Software migration without interruption: rebuilding in parallel, week by week

An uninterrupted software migration is not decided at six on a Monday morning. It unfolds over weeks, keeping the old tool alive while the new one is built alongside it, with a period maintaining both. Here is the week-by-week plan.

An uninterrupted software migration keeps the old tool running while the new one is built alongside it, then switches one activity at a time. A single weekend switchover is the scenario that breaks: nobody knows the old system well enough to predict Monday's missing pieces. Running both costs data-entry hours, the price of staying open.

Week zero: write what the old tool does

Before building, someone must state what the current tool does, including what it does badly. A takeover starts with what exists: agents read the software and return a French assessment of screens, data, and processing, including areas they did not understand. You correct it, and those corrections are worth more than the document because they contain what was written nowhere. We described what AI changes in rebuilding ageing business software; the difference lies in that prior reading, not code-writing speed.

The first weeks: build alongside, connect nothing

The new tool is built read-only. It displays copied real data and writes nothing anywhere. Nobody uses it for work. You open it every other day looking for missing details: the column the old tool coloured red, the field your assistant always fills last, the rule sending Friday orders on Monday. Every detail found now costs a correction; found after switchover, it costs a day's activity.

The parallel period: one activity at a time

Choose the least risky, most frequent process: quotes are a good first candidate, invoicing a bad one. For two to three weeks, the same person performs it in both tools and you compare results each Friday. Settle three questions: who enters twice, until when, and who decides whether a discrepancy is a new-tool defect or an unwritten old-tool habit? Once the process runs without discrepancies, it leaves the old tool permanently and you move to the next. Invoicing goes last because it leaves records you must justify.

This period has a cost nobody likes budgeting: two to six weeks of one person entering everything twice. Pricing it beforehand prevents the discouraged mid-project suggestion to stop duplicate entry 'for this week', losing the comparison that justified the exercise. Also plan who answers customers then, because the tools will produce different quote numbers and someone must explain by phone.

The data follows along

Migration happens in two stages: a complete copy for building and checking, then a fresh copy before each activity's switchover. Fixed data, invoice history, closed contracts, remains readable in the old tool while kept open, avoiding moving everything at once. Cleanup happens before transformation, never afterward: migrating from a spreadsheet follows the same rule and carries the same penalty for forgetting it.

Decide rollback in advance

Before the first switchover, write what would send you back: doubled processing time, an incorrect invoice, three people blocked the same day. While the old tool stays on, returning costs a morning. In the new tool, every approved milestone leaves a restorable version, code and documents together, without manual undoing. That safety net helps only if the decision to use it was made calmly, before fatigue and lateness.

The one-page plan

List processes from least risky to most consequential, with a switchover date and end date for duplicate entry beside each. Name whoever settles discrepancies. Keep the old tool readable for three months after the last switchover, telling the team so nobody resumes working there. If the page fits ten lines, migration is scoped; if forty, split it further. Rebuilding in one block remains the most expensive path.