Thank you Steven for looking at my request and implementing a solution, if only all developers could be so responsive!
Just so I understand correctly: to replace an mp3 with FLAC, for example (i.e. not using the file converter), I would add the FLAC files to the library, then use the hot key to copy the tags including playcount etc. from each mp3 file and paste onto the corresponding FLAC. Then once finished for all the tracks, delete the mp3 files.
Sounds straightforward enough, obviously I'll try it out once I get chance.