Start with dependency truth
Legacy software is rarely one thing. It is code, data, undocumented rules, scheduled work, external integrations, user habits, reporting assumptions, and operational knowledge held by a few people.
Map critical paths and ownership before choosing a replacement technology. The most important dependency is often not the one visible in the architecture diagram.
Create stable seams
APIs, events, adapters, identity boundaries, and carefully chosen data contracts can isolate change without pretending the old and new worlds are independent.
Each seam should have an owner, observable behavior, compatibility rules, rollback, and a measurable condition for retiring what it replaces.
Modernize by business value
Sequence work around customer impact, operating risk, delivery speed, and the ability to learn. A smaller change that creates future options can be more valuable than a large rewrite that delays evidence.
The objective is not newer technology. It is a technology estate the business can change with greater confidence.