Measuring What Legacy Modernization Actually Costs

Most modernization projects are scoped around the cost of building the replacement system. Fewer are scoped around the cost of the transition period itself — the months or years where the old and new systems run in parallel.

That parallel period carries its own engineering cost: dual maintenance, data reconciliation, and the operational discipline required to keep both systems consistent. Underestimating it is one of the most common reasons modernization timelines slip.

A more realistic plan treats the transition period as its own phase of work, with its own budget and its own success criteria — not as a rounding error between 'old system' and 'new system'.