@stshank Starting from scratch is certainly the easiest.

Most importantly, as I'm sure you realise, you "add" the new drive and retain the old drive in your TM settings, that way you get to keep your history for as long as needed.

On the third hand you can periodically plug in the old drive to have redundant backups, even if the history is pretty truncated.

My primary TM disk is a network drive, but my secondary is a USB disk which I plug in once a month and otherwise keep stored in a safe place.

@stshank

100% concur. It feels like a deep "miss" in UX. The whole purpose of preserving arbitrary file-history depth relative to total available storage, without being able to move that depth to larger storage, is fundamentally broken. Since this used to be possible pre-APFS, I suspect it's a use case that was missed (since you can use any drive with Time Machine).

@darkuncle