I had good success with this flow for major changes (both in-house as well as contributing to FOSS). The target was clear & discussed upfront (sans details that

I had good success with this flow for major changes (both in-house as well as contributing to FOSS). The target was clear & discussed upfront (sans details that pop up during the actual work), and achieved by a series of consecutive PRs. It's important to be constructive, communicate the intent properly and give the other party enough time to digest the info and think properly about it. Obviously this also depends on the maintainers and their mentality.