As promised, I am reporting back from Tech Support on trying to organize my hot mess of prior BIAB installs (2010, 2014, 2019...) and associated Styles, RealTracks, RealDrums, ... spread over several drives.

Cut to the chase: they recommend waiting for UltraPAK 2025 in Fall which will have the downlevel stuff, and be done with it.

Copying files/folders likely to end up missing things.

But until the UltraPAK, they said wipe my new hard drive clean. Then install my 2010 (!) "EverythingPAK" from its PG Music hard drive onto the clean drive.
Then install 2014, 2019, 2024 BIAB on top of it.

So there ! Thanks to all for the suggestions.

Best,
Bob