Legacy System Modernisation for Scaleups

Depends on monolith size and your risk tolerance. A 50k-line codebase with clear domain boundaries might take 6-9 months if you're willing to run old and new in parallel. A 500k-line tangled system could take 18-24 months. The key is that you don't block shipping during that time. We plan in phases: extract a low-risk domain first (often payments or notifications), validate the new architecture, then expand. Each phase ships feature value while reducing legacy surface area. We give you honest estimates after a two-week code audit.

We run shadow traffic tests months before cutover. Live user data flows through both old and new systems, and we compare the results. If responses diverge, we find and fix the bug before any customer sees it. On cutover day, we keep the old system running for 72 hours as a read-only fallback. All transactions are logged so we can replay them if needed. We also run daily snapshots of production data in a staging environment that mirrors the new system exactly. If something unexpected happens, we revert to the old system with zero data loss in under an hour.

Short term, slightly. You'll dedicate some engineering capacity to the new architecture. But we work to unblock your main team so they ship into the new system as soon as its first domain is stable. You'll see the trade-off: maybe two weeks slower on features month one, but then faster deployments, fewer rollbacks, and easier onboarding for new hires. By month four or five, you'll ship faster than before because you're not fighting deployment friction and architectural limits. Scaleups we've worked with typically break even around month six and ship 20-30% faster by month nine.

Yes, that's the model we use. We don't take over the codebase and disappear into a cave. Your team and ours work in the same repo, pair on critical refactors, and ship code together. You'll have a designated senior engineer from our team embedded in your sprint cycle who reviews designs and helps your team understand the new architecture patterns. This also means knowledge transfers naturally instead of us handing over a black box you don't know how to maintain.

We expect this. Early phases of modernisation always reveal assumptions that don't hold up. That's why we run discovery and proof-of-concept work at the start. If a new constraint emerges (regulatory requirement, unexpected traffic pattern, acquisition integration), we replan. The beauty of phased migration is you can adjust direction without scrapping months of work. We've moved from microservices-first to modular monolith and back again based on what the data showed. Cost you a few weeks, not a full restart.

We structure the work so handoffs minimise waiting time. Your UK team has direct access to our Leicester engineers. On complex problems that need real-time collaboration, we schedule overlapping hours. For async work (code review, documentation, testing), the rotation means something is always progressing. For a scaleup, this is an advantage. You get weekend coverage and weekend resilience without paying premium rates. If your main team finishes work at 6pm UK time, our Spain or Ukraine team picks up the next phase and ships updates overnight.