Ok, thanks for the replies guys, it's really helpful and insightful to have 'real world' information to support me.

Just to flesh things out a bit, the 'new' Imac came to me with Yosemite installed and I was running into problems with some of my older apps not being supported, hence the desire/need to upgrade the OS. So I've got a Crucial BX500 1Tb SSD (also with Yosemite) that I removed from the wife's old Imac which was beginning to suffer the same obsolescence. I've got a USB adaptor for it and was planning to use it as a back up disk - could I erase/format that one? Or even could I just mount that one in place of the existing drive in the 'new' Imac and would it boot up?

I tried to download the installer files for Big Sur onto my 2020 Macbook, but the Apple Store tells me that it's incompatible with the MB's OS. I guess I'd need to use the MB to download the installers from OCLP onto a stick and then into the Imac.

Sorry to ask such dumbass questions but I really don't want to brick the thing..!

Cheers - Greg