It seems a very helpful feature is missing, and implementation should be simple:

An existing screen was altered and now I want to save it, so the opened screen is simply being overwritten, perhaps with a popup shown 'do you really want to overwrite the existing screen?'.

Currently, I have to save the altered version as a different screen, keep in mind or redo the description given, and then delete the previous/obsolete screen, which is overly complicated. Thanks

This is a common request. Our concerns with allowing screens to be overwritten relate to versioning and ensuring consistency in results from identically named shared screens. In the future, we may allow private variables/screens to be overwritten (with a warning dialog like the one you propose) while disabling that functionality for shared variables/screens.