Thanks for all your help with the separate ticket, and in case any others are interested, I've learned two things:
1. To create a Smart Copy with an alternate recording, simply delete the original recording in the Smart Copy and add the alternate recording. (No need to worry about side effects of deleting the original recording from the smart copy). The alternate recording will be played by the existing automation for the smart copy song. Perfect for my use case of a 'practice version'.
2. If you replace the contents of an existing Recording by clicking on that recording and uploading a different file, it replaces that Recording 'object' wherever it is referenced, even if referenced by multiple songs, copies, etc. If you really want a different recording in Song-B (but not affect some other Song-A that also references it), you need to delete the recording in Song-B and add the alternate recording.