Running the old system and the new one side by side

When you replace a system your business runs on, the risky day isn't the day the new one is finished. It's the day you switch the old one off. Up to that point, almost everything can be undone. If that day goes badly, it can't.

The safest way through it isn't exciting. You run the old system and the new one side by side for a while, and you only switch off the old one once the new one has proved itself on real work. This is how that goes in practice, and the few decisions that make it go smoothly.

Why not switch over in one weekend?

Doing it all in one weekend is tempting. It's quick, it feels decisive, and you avoid paying for two systems at once. But it bets the business on testing having found everything, and testing never finds everything. The surprises are in the everyday work: the customer with the odd billing arrangement, the report finance runs once a quarter, the workaround a long-serving employee has used for ten years and never mentioned.

Running both systems for a while gives those things a chance to show up while the old system is still there to fall back on.

Decide which system holds the official record

The first decision, and the one people skip most often, is which system holds the official version of each kind of record while both are running. If both systems let people edit the same customer, they'll disagree within a week, and someone will spend a month sorting it out.

So before the overlap starts, we write it down, one type of record at a time. New orders might go only into the new system from a set date, while old invoices stay in the old system, where people can look at them but not change them. Inventory might stay with the old system right up to the last day. However you split it, everybody knows the split, and the system that doesn't own a record only lets people look at it.

Copy records across, never back

While both systems are running, records should be copied from the system that owns them to the one that doesn't, on a known schedule, and never back the other way. Letting changes flow both ways between an old system and a new one sounds convenient. It's also where most of these projects get into trouble, because it doubles the ways a record can end up wrong and makes it hard to tell which side made the mistake.

Moving years of old records into the new system is a project of its own, with its own traps. We wrote about that separately in moving decades of data.

Compare the two on a schedule

Running both systems only helps if someone compares them. Pick a handful of numbers that matter to the business, such as the daily totals, the open orders and the invoice run, and check them against each other on a regular schedule. Every difference turns out to be a mistake in the new system, a mistake in the old one, or a step in the process that was never written down. All of them are worth finding before you switch, and the last kind is the most common.

Agree up front when you'll stop

Without an agreed finish line, running two systems can drag on for months, with staff doing everything twice and patience wearing thin. So we agree at the start what has to be true before the old system goes off. Usually that means the numbers match across a stretch of normal business, including at least one month-end, and the people who use the system every day say they'd rather not go back.

Once that's true, the old system is retired. We don't delete it. It stays available as read-only, so people can look things up in it, for reference or for an audit, for as long as anyone needs to.

What it costs

Running two systems isn't free. People enter some work twice, and for a while you have two systems running. But you know roughly what that costs and when it ends, which you can't say about a weekend switchover that goes wrong. If that happens, the cost is whatever the business loses while nobody can take an order, and no one can tell you that number in advance.

That's also why our legacy modernization work gives you the new system in pieces your team can start using as each one is ready, rather than one big switch at the end. Each piece gets proved on real work while the old system keeps handling the rest. If you're facing a replacement and dreading the switchover, talk it through with us. We start planning that last day in the first conversation.