When a legacy system is replaced by a new system, the maintenance of the legacy system is halted. This is called the freezing of the legacy system. This is necessary because, when you build a new system, you need to replace all the functionality of the legacy system. If you try to modify and extend the legacy system at the same time, you are shooting at a moving target.
The problem is that most legacy systems are essential systems for the organisation. Freezing them for a short period is not a problem. But to do so for a longer period may be a major problem.
Commercial software
For commercial software products freezing the current application means losing market share. Companies can only afford a short freeze period, because when the period is too long, new customers will prefer the products of competitors. This may start as a slow process but could gain momentum.
You can try to halt this trend by showing customers your nice new software which they can expect ’soon’. But if ’soon’ is not soon enough or as promised, the company is in dangerous territory. At that point it is even dangerous to show your customers the new software because they will realize they have to relearn the new software, and if that is so, they may consider changing their software supplier. The management must now juggle with promises, customers and the functionality of the legacy system.
Defreezing the legacy system
At a certain point it becomes clear to everybody that building the new systems takes too much time. So the legacy system can no longer be frozen. The maintenance effort of the old ICT system must start again.
This is a very expensive moment for the following reasons:
1. At that point there is a backlog in maintenance so a lot must be done in a very short time
2. All new functions must also be included in the new system but this is not a one-to-one operation because some functions are already incorporated in the new system, but in a different way.
3. Synchronization and control of the maintenance effort in the old ICT system and the new system will, from that moment, take a lot of project management time and effort.
