Hmm, seems that the function was achieved in MediaMonkey through a 3rd party script written for it, so it isn't a built in feature.
The alternative method I settled on using is:
1. Use Mp3tag to copy the tags of the original files in my library.
2. Then Mp3tag to paste tags to my replacement files.
3. Open MB and delete the original files from My Library and My Computer.
4. Last step is to Rescan/Add the new files into my library, Folder Reorganize them, and finally synchronize their play count with Last.FM (because Mp3tag doesn't copy the play count but MB works beautifully with Last.FM)
Certainly not a quick method but I only have 15-20 CDs to do it for, I can only pray for those with 100s or 1000s to do
. But in the end it works because Mp3tag correctly preserves tags like Ratings and Guest Artists in the Multiple Artist Splitter and as far as I can tell the only tag not copied and pasted is the play count.