Legacy systems are rarely bad systems — they're systems that outlived the assumptions they were built on. They still run the business, which is exactly why replacing them is risky: the lights can't go off while you rewire the building.
Phased over big-bang
A phased approach — modernizing in slices, keeping the old and new running side by side, moving load across deliberately — trades a dramatic launch for something far more valuable: continuity. The business keeps operating while the technology underneath it changes.
It's slower to look finished and much harder to get badly wrong.
Understand before you replace
The most dangerous part of any legacy system is the behaviour no one documented. Modernization has to begin by understanding what the old system actually does — including the edge cases it quietly handles — before anything is switched off.
