Legacy systems are not the enemy. They are the encoded business logic of a company that survived long enough to accumulate them. The risk is never the old code itself — it is changing it without understanding it.
The most reliable path is the strangler pattern: stand up the new system alongside the old, route a sliver of traffic through it, and expand coverage as confidence grows. The legacy system keeps running until the day it carries no traffic at all.
This keeps the business continuous, makes every step reversible, and turns a terrifying rewrite into a sequence of small, boring, low-risk deployments.
Before touching anything, we characterize behavior with tests against the live system. Those tests become the contract the new implementation must honor — so modernization never silently changes what the business depends on.