With regards to updating existing records, unfortunately we don't currently have good tooling & support for this, so you may need to record new sets of sessions as your application changes. I would suggest starting off with testing a few core flows.
This reminds me of the test suite for one of my projects where E2E tests only covered relatively simple scenarios because of subpar WS mocking support, leaving more important, complex interactions to manual (can't run in CI) or fully integrated (expensive to author and run often) testing. The situation changed only after we wrote a custom WS mocking layer over the HTTP mocking the framework provides, yielding a dramatic increase in coverage. Out of dozens of developers I interviewed, only a few solved this issue to some degree. Clearly, mainstream testing frameworks provide insufficient support for the use case.
But I agree that it would enable replay for a lot of very interesting, complex projects. I've worked on some FX trading UIs that have been challenging to test without standing up a lot of backend services.