I have an old 2007 iMac with an old OS on it, something like Mountain Lion. I have backed it up on a Time Machine disk.

I have a new (to me) 2012 MacMini I have just bought which I think has High Sierra OS (or at least something more recent than the iMac) and I want to put all my stuff on that.

I read this guide on how to do it: https://support.apple.com/en-gb/HT201250

Thing is I am not clear whether when I use Time Machine to transfer if it is going to wipe the OS on the MacMini and put the old Mountain Lion on and then I might get stuck with that – or can I upgrade back to High Sierra.

Or is another approach to transferring my stuff a better approach to start with a cleaner slate?

Any advice appreciated!

Just read a bit more and it looks like Migration Assistant might be a better bet as I can transfer all the files and applications without wiping the nice new OS!

Yes to Migration Assistant.

Another vote for Migration Assist..

Thanks both 🙂