Replatforming a SaaS without a full rewrite.

A founder's hub on migrating a legacy SaaS to a modern stack, on adding AI to a product that already has users, and on what regulated-industry constraints do to the budget. Three essays cover the framework, the cost reality, and the regulated-case math.

The framework

The full rewrite is the most expensive way to fail at replatforming. The cliche from the early 2000s still holds: most full rewrites miss the deadline by a year or more, ship something customers reject, or get killed before completion. The replatform that ships keeps the old stack live, runs the new stack alongside it, and migrates traffic in deliberate gates over months. Four moves compose the framework, in this order: strangler at the edge, parallel data, contract tests on the old stack, and gated traffic ramps.

The cost reality

The other half of replatforming is honest pricing. Adding AI to a product that already has users runs €25K to €80K in a senior shop, six to twelve weeks. Below €15K and the work is either a Streamlit demo or a wrapper around an LLM API. Above €100K and you are paying for an org chart, not the integration. The essay walks the four shapes (RAG, agentic workflows, evaluation harnesses, vendor migration), the six factors that move the price, and the four parts of every project that founders consistently underestimate.

When the industry changes the budget

The same AI integration scope that runs €25K to €55K in an unregulated SaaS lands €15K to €60K higher in HR, finance, healthcare, or legal. The uplift is real engineering and contract work for data residency, audit logging, deletion guarantees, model attribution, and compliance review. The contract chain between buyer, build vendor, and model provider decides whether the engagement can ship at all; the cost work is downstream of that. The essay covers the contract structure, what audit logging actually entails, and the four contract clauses that matter most.

How I work on replatforming and AI builds.

The shape of the engagement depends on which part of the arc you need help with.

For AI integration specifically (RAG, agentic workflows, evaluation harnesses, model migration), the work fits a fixed-scope build engagement: €25K to €60K, four to twelve weeks. That price is honest because the scope is bounded.

For an actual replatform arc (a six-to-eighteen month migration of a legacy SaaS), fixed-scope is the wrong shape. The full arc is best run with your existing team executing, the studio coming in as a fractional CTO retainer that owns the architecture decisions and reviews each per-route migration, and the studio occasionally taking a single bounded build engagement (the strangler at the edge, the contract-test build-out, an AI bolt-on) inside the arc. That keeps the price honest at each phase and avoids the trap where one vendor owns the whole multi-million-euro arc and the buyer cannot course-correct mid-flight.

The Relokate HR platform replatform between 2024 and 2025 is the closest case study; the technical anatomy is on the platform page.