The old system is to go; the new one has been chosen or is being built. Now comes the question that decides success more than the choice of system: how does the business get from here to there? All at once, on a single day - or piece by piece, while the old system is still running?
Both routes are common, and both have advocates with good arguments. One is over quickly and needs no interfaces between old and new. The other is calmer and leaves the way back open. This article compares them so that you can discuss the question with your vendor on an equal footing. If you are still unsure whether the old system really is at its end, start there.
The cut-over date
With a cut-over date the old system is switched off on a fixed day and the new one switched on. In the evening the last document goes into the old system, the data is migrated in between, and the next morning the first document goes into the new one. This is often called a “big bang” - everything at once, with a bang. The term is apt: there is no going back, and whatever was not prepared shows up the next morning.
Step-by-step replacement
With a step-by-step replacement one area after another moves into the new system: first complaints, then order entry, then the warehouse. The old system keeps running for the remaining areas. For a while the two systems work side by side, connected through an interface where possible so that nothing is entered twice. Once the last area has moved, the old system is switched off - and nobody misses it.
Both routes compared
| Cut-over date | Step by step | |
|---|---|---|
| Risk | high: if something fails, the whole business stands still | limited: at most one area is stuck, the rest keeps running in the old system |
| Parallel operation | none, but intensive preparation instead | yes, with an interface or coordination between old and new |
| Training | all staff at once, shortly before the date | area by area, the experience of the first helps the second |
| Data migration | once, complete, under time pressure | per area, without haste, with trial runs |
| Way back | practically none, the old system is off | open: an area can go back, the old system is still running |
| Interim solutions | none | interfaces and transitions that disappear again later |
When a cut-over date does fit
The cut-over date has a poor reputation because the big failures you read about were almost always cut-over dates. Still, there are situations in which it is the better route:
- The system is small. An application with one workflow and a few screens can be switched on a single day. Step by step would be more effort than it is worth here.
- Few users. Five people who all sit in the same room learn the new system together and help each other.
- Accounting. Accounting systems are switched at the turn of the financial year, with the closing in the old and the opening in the new. A financial year split across two systems pleases nobody, least of all the tax adviser.
- No way to run in parallel. Some old systems have no interface and will not accept one. Then parallel operation means double entry, and the cut-over date is the lesser evil.
How step-by-step replacement works in practice
- Choose the first area. It is the area under the most pressure: where most of the work runs in Excel today, where customers wait longest or where the old system hangs most often. Not the easiest one - the business has to feel that it is worth it.
- Connect old and new. As long as both systems run, they need a connection: customer data from the old system into the new, orders from the new back for invoicing. Through an interface where possible, otherwise through a regular export. This is effort that disappears again later, and it is still well spent.
- Move area by area. Each area gets its own data migration, its own test environment, its own training. What the first area taught makes the second easier.
- Switch the old system off gradually. First read-only, then locked for individual departments, finally altogether. The moment nobody asks where it went is the right one.
For Access databases and other applications that have grown in-house this is almost always the better route, because the many small rules become visible area by area instead of all being missing on one day.
What both routes need
The route decides pace and risk, not the homework. Three things are needed either way:
- A planned data migration. Which data comes along, which stays in an archive, who cleans it up beforehand? On a cut-over date all at once, step by step per area.
- A test environment. A version of the new system with real, migrated data in which the department runs through the workflow before it goes live. Whoever switches without one is testing on the live business.
- A contact person in the business. Someone who knows the workflow, has the authority to decide questions and is reachable after the switch.
Questions before you decide
- What happens in the business if the new system does not run on the first morning?
- Can the old system be connected to the new one, or would parallel operation mean double entry?
- Which area is under the most pressure today - and could it be switched on its own?
- Who trains the staff, and how many at a time?
- Is there a date that dictates the cut-over, such as the turn of the financial year in accounting?
We replace old systems step by step, because the way back stays open and the business keeps running. What that looks like in detail is on our page on replacing old systems.
