Thanks Matt - I did approach support initially, and they said that as I was still within the 30 days for the 2024 purchase, I could claim the return and refund which I could then apply to the new purchase, but only of the upgrade as for me, 2024 was an upgrade to 2023 USB Drive version. She made no mention of just deactivating licences, however even if I did I suspect it would make no difference.

The time to do Musocity's plan is not daunting time wise, more a case of I'd probably muck something up and then not be able to use it properly. And as I would also need to format the USB Drive to APFS (for optimal use with MacOS 14) I'd then be risking losing everything on it, with no guarantee of success, in case I ever wanted to (but I seriously doubt it) return to Microsoft.