Can an existing learning path be converted to a mandatory learning path, or the other way around? Is the learning progress retained?

Yes, there is a button directly in the first section of the learning path creation, which allows assigning the learning path as a mandatory learning path. Existing mandatory learning paths can also be changed to normal learning paths that way.

Change from LP (learning path) to MLP (mandatory learning path) (or vice versa): learning progress remains the same.