[Geoff Bacon]

I think your best bet is to adopt the new name as the master file.

Then create a song with the "original" file name which simply contains text to remind you the name of the master song.

I play in 3 groups that have several common songs.
To avoid confusion, and to allow the groups to diverge, each group has its own copy of the file (i.e. 3 separate files).
To differentiate between these files, I append a short mnemonic to the end of each file name eg. " - MUJ", " - Pub"
I keep each groups files in their own collections.

Remember that a song can be in more than one collection i.e. you can put both master and "reminder" songs in both collections 

Cheers
Geoff