Replacing an old system without shutting the business down

The most common request we get is not "build something new" — it is "replace this thing we cannot live without." An old accounting system, a database someone built years ago, a site whose developer stopped answering.

The instinct is to rebuild everything and switch over one weekend. That is usually where these projects go wrong. The old system has years of small rules baked into it that nobody documented and everybody depends on.

A safer approach is to replace it in slices. Put the new system in front, let it handle one function at a time, and leave the old one serving everything you have not moved yet. Staff keep working throughout.

The part people underestimate is checking parity. For every function you move, run both versions side by side and compare the output before switching the old one off. It feels slow. It is much faster than discovering in month three that your invoices are wrong.