Computerizing is a lot more time-consuming and costly than most contractors imagine. Transferring data takes a lot of time, often months, and inevitably involves transcription errors. Managing a fail-safe transition is costly. It always takes longer to install, transcribe, and debug a system than anyone projects. Conversion often costs as much as two or three times early estimates. It's risky to depend on a new computer system too soon. You need a formal contingency plan in case data or the system is lost. Professional advice and even insurance is available to cover the contingency. Remember to store duplicate data off the premises to further protect yourself.
Run your old system, be it manual or an old computer system, as the primary system during the entire transition period. Abandon it only after the new system generates the same results as the old one for 2 months in a row. Even if the manual system is overburdened and running late, it is the method that has worked so far. Keep it operational until you're confident that the new system gives you the same results as the old, with the same raw data.