Project Audio Folder - change in existing project

Yes, you can do this, but it's a bit of a chore. Hope you don't have too many.

Use Options->Global->Audio Data to select per project audio folders.

Open the file that you want to change. Do File->Save As and select Copy all audio with project, then give the new path for the project, then Save.

You'll now have two versions of the same project, one with per project folders, one without. After making sure this has worked, you can get rif of the old one.