Stabilization
Why Stability?
Many systems can suffer from instability and fragility. Legacy systems are particularly prone to these problems for two main reasons:
Complex systems require maintenance, and without it, the system degrades.
Continual modifications can add complexity and inconsistency.
These can be viewed as a kind of entropy (I know physicists are yelling that it's not really entropy but IT systems do tend toward disorder unless work is performed) and the longer a system exists, the greater the effect. If you're lucky, then your legacy system will have received all the maintenance required, but then you wouldn't have bought this book.
I would always suggest performing a stabilization phase when working with a legacy system – even if you intend to replace it. If the system is stable, then you will not be constantly pestered by user complaints.
Note
Stabilizing a system due to be replaced is not wasted effort, as it removes distractions
Not only will a stable system reduce your distractions...