Converting Apple Music library without duplicating files

Gabriel,

Are you talking about a format conversion such as WAV to MP3? Music/iTunes (and all the 3rd party tools that I have used) will create a new version and keep the original intact.

It is easy enough to tell the new copy from the old copy by looking at the Kind column in your library, and then you can delete the one you don't want. Would that work for you?

Yes, that makes sense, since the file types you mentioned are the bulkiest ones (except for AAC). It will be a long process any way you slice it. You can launch multiple conversions at once, but they execute serially. Then you can do the extra step of deleting the original. Do a small batch and see how it goes. Good luck!

I don't see a way to avoid it.

What, if I might ask, is prompting you at this time to convert the whole 15,000 track library to a different format?