Converting library to Windows compatibility - keep separate from legacy version?

If I convert a library to Windows from Legacy, and publish it, what happens to the existing versions? Are they still available to Legacy projects, or will it now be seen as a Windows library and not shown to Legacy projects in Manage Packages?

How is everyone handling the code branch? If I want to add features or fix something, I assume I’ll now have to fix it twice - once in the Legacy version and again in the Windows version. But that means I need to have them as separate libraries with different names, right?

I’m thinking I’ll just copy the project to a different folder with Windows in the name so we know the difference, then change the name in project.json to also include Windows so it publishes as a separate library. Thoughts?