Legacy systemmodernization services without stopping the business.

We take over aging web and mobile applications, find out what's really inside them and move them to a modern, maintainable stack while your users keep working. No big-bang rewrite unless it truly makes sense.
Assess your system

Your old system still runs the business. That's exactly why it's risky.

Legacy software usually works well enough to keep, and badly enough to slow everything down. Application modernization is about removing that tension: keeping the business logic that matters, replacing what holds you back, and doing it in small, reversible steps instead of one risky switch-over.

What makes legacy systems hard to change

01

The people who built it are gone

The original team left years ago. Documentation is thin, and the real rules live only in the code.
02

Every change breaks something else

No tests, tight coupling and side effects nobody can predict. Simple features take weeks.
03

Outdated frameworks and libraries

Dependencies are no longer supported, security patches stopped and upgrades are blocked by other upgrades.
04

Hiring for the old stack is painful

Few engineers want to work on it, and the ones who can are expensive and hard to keep.
05

Infrastructure nobody wants to touch

Servers configured by hand years ago, manual deploys and no clear picture of what runs where.
06

Rewrites that never finish

A previous attempt to rebuild from scratch ran out of time or budget, and now two systems need care.

Signs your application needs modernization

One of these is a warning. Several at once means the cost of waiting is already higher than the cost of acting.

1

Releases have slowed to a crawl

Changes that used to take days now take weeks, mostly spent on testing and fixing regressions.

2

Security updates are overdue

Your framework, runtime or operating system is past its support window.

3

Users complain about speed and design

The app looks dated, feels slow and loses customers to newer competitors.

4

Integrations are getting harder

New partners, payment providers or mobile apps can't connect without custom workarounds.

5

Hosting costs keep climbing

Old servers and licenses cost more each year, and scaling means buying bigger machines.

6

Store or platform rules are catching up

Mobile apps built on old SDKs face store requirements they can't meet without serious work.

What we do

Legacy app modernization services

Legacy system assessment

Code, architecture, infrastructure and data reviewed, with a prioritized list of risks and a modernization roadmap.

Refactoring and stabilization

Tests around critical flows, cleaner modules and fixes for the bugs that hurt users most.

Framework and runtime upgrades

Moving to supported versions of your languages, frameworks and libraries without breaking production.

Incremental rebuilds

New modules replace old ones one at a time behind the same interface, so users never face a hard cut-over.

Cloud migration

Moving from hand-managed servers to AWS with Docker, automated deploys and monitoring.

Data migration

Cleaning, mapping and moving data to a new schema or database, with checks before and after the switch.

Mobile app modernization

Rebuilding the architecture of aging iOS and Android apps and bringing the design up to date.

UX and UI refresh

Redesigning dated screens and flows as part of the modernization, not as a separate project.

Four ways to modernize, one decision at a time

Not every part of a legacy system needs the same treatment. We recommend a path per module, based on its value and risk.

Refactor in place

For code that works but is hard to change

Tests first, then cleaner structure

No change for users

Lowest risk, quickest payoff

Replace module by module

New services built next to the old system

Traffic moved over feature by feature

Old code retired when nothing uses it

Business keeps running throughout

Move to the cloud

Applications containerized with Docker

Infrastructure on AWS, defined as code

Automated builds and deploys

Monitoring from day one

Rebuild from scratch

Only when the old code can't be saved

Scope based on how the system is really used

Data migration planned up front

Parallel run before switching over

Who is this for

For companies whose software outlived its stack

Product companies

Your core product runs on an aging codebase, and the roadmap is slowing down because of it.

SMBs

A system built years ago still runs operations, but nobody is left who fully understands it.

Startups after a fast MVP

The first version got you customers. Now its shortcuts block growth and fundraising due diligence.

Owners who inherited software

You took over a business or a product and need a clear, honest view of what the software can and can't do.

What's your main goal?

Understand what you have

An assessment and a clear roadmap.

Stop the fires

Stabilize the system before anything else.

Ship faster again

Refactor the parts that slow the team down.

Cut hosting costs

Move to modern cloud infrastructure.

Refresh the user experience

New design on a cleaner foundation.

Replace the system

A planned rebuild with a safe switch-over.

How a modernization project runs

01

Assessment

We review the code, infrastructure, data and integrations, and talk to the people who use and support the system.

02

Roadmap

You get a prioritized plan: what to refactor, replace, migrate or leave alone, and in which order.

03

Safety net

Automated tests, monitoring and a deploy pipeline go in first, so every later change is measurable and reversible.

04

Incremental delivery

We modernize one module or flow at a time and release it to production, keeping the old and new parts in sync.

05

Cut-over and retire

Data is migrated and checked, traffic moves to the new parts and old code and servers are switched off.

06

Hand-over or ongoing support

Documentation and knowledge transfer to your team, or we stay on for maintenance and new features.

How we work

Two ways to begin

Assessment first

most popular

A 2-week fixed-price Discovery Sprint

Fixed price. Modernization quoted after it.

For systems you don't fully understand yet, or when you need a plan to present to stakeholders.

Review of code, infrastructure and data

List of risks ranked by impact

Modernization path for each module

Phased roadmap and estimate

Yours to keep, whoever does the work

Start with an assessment

Modernization team

Engineers, QA and a PM on your system

Monthly, team-based.

For teams that already know what needs to change and want steady progress alongside daily support.

Engineers who take over the codebase

Stabilization and modernization in parallel

Releases to production every sprint

Progress and risk reports

Knowledge transfer to your team

Discuss your system

Systems we've modernized

subscription app

Flight Tracker+ — iOS subscription app

USA
Global
59%
growth of active user base
Before
→
after
↓
Rating declining
↑
Revenue €200K → €1.2M in 6 months
↓
Revenue stagnating at €200K
↑
Stable weekly release cadence
↓
Legacy codebase, original team gone
↑
Architecture rebuilt
↓
No clear path to recovery
↑
Clear path for future growth
Real estate

Modernizing a Multi-Service Real Estate Marketplace

Read the case study →
Pharmacy · DACH

Redesigning and modernizing an Online Pharmacy mobile app

Read the case study →

Make the most of the experienced team

We are a global team of
creative
and
strategic thinkers
who specialize in building custom digital products
from scratch. Our main goal is to
deliver high-quality solutions
that meet our clients' needs.
We are a global team of
creative
and
strategic thinkers
who specialize
in building custom digital products
from scratch. Our main goal is to
deliver high-quality solutions
that
meet our clients' needs.
Learn more
85%
recurring customers
12M
the biggest fundraising round by one startup
X3 MRR
growth of partner's business in 5 mos
2 weeks
for project start
5+ years
av. partnership duration with SMBs and product companies

Why Gilzor

We regularly take over code that other teams left behind. Modernizing it calmly, without putting the business at risk, is a large part of what we do.

What we bring

Experience with inherited code

Rescuing troubled codebases and apps whose original teams are gone.

Web and mobile in one team

Back end, front end, iOS and Android modernized together.

On-time delivery

98% of our projects are delivered on time.

Quick start

We can usually start in about two weeks.

Modern target stack

React, Node.js, Python, PostgreSQL, AWS, Docker and Kubernetes.

What makes us different

No rewrite by default

We rebuild only what can't be saved, and say so plainly.

Business keeps running

Changes go out in small releases, not one risky weekend.

Safety net first

Tests and monitoring come before big changes.

Long-term partner

Average partnership with SMBs and product companies is 5+ years.

Overlap with US hours

Our engineers in Europe overlap with part of the US working day.

What our clients
frequently ask

Should we rewrite our legacy system from scratch?

Usually not all at once. Full rewrites carry the most risk. In most cases we refactor what still works and replace the rest module by module. If the assessment shows a rebuild is the right call, we plan it with a parallel run and a data migration path.

Can you modernize a system without documentation or the original team?

Yes, that's a common starting point. We read the code, trace the data, test real flows and interview the people who use the system. The assessment ends with documentation you didn't have before.

Will our users notice downtime during modernization?

We plan each step to avoid it. New parts run next to old ones, traffic moves gradually and every release has a rollback plan. Any planned maintenance window is agreed with you in advance.

What technologies do you modernize to?

Typically React or Next.js on the front end, Node.js or Python on the back end, PostgreSQL, MySQL or MongoDB for data, and AWS with Docker for infrastructure. For mobile, Swift, Kotlin or React Native.

How is a modernization project priced?

We start with a fixed-price Discovery Sprint that ends in a phased plan and estimate. After that, phases can run as fixed-price projects or with a dedicated team billed monthly.

Can we keep shipping features during modernization?

Yes. We usually split work so new features and modernization share the same releases. Often a feature is the reason to modernize the module it touches first.

No sales pitch

Tell us about your legacy system

What does it run, what stack is it on and what worries you most? We'll suggest where to start and how to keep the business running while we change it.

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