Rewriting a working system from scratch sounds clean, but it usually takes longer than planned and delivers value only at the very end.
Find the seams
Look for parts of the system that can be wrapped, replaced or exposed through an API without touching everything else. Those seams are your safest starting points.
Deliver value every phase
- Pick one painful workflow and modernize it first
- Run old and new side by side where possible
- Retire old parts only after the new ones are proven
Each phase should leave the business in a better state than it found it.