I have an older RB2011Ui that has been working great - and I’ve bought a RB4011iGS to replace it. Is there a way to “upgrade” via a config backup so that I don’t have to configure everything on the new router (besides activating the 5GHz, etc) - or should I start afresh?

Well, you should start afresh, however, you can see what you've done via: /export compact file=MyFile.rsc

I have never used config backup method successfully but I do use the config export method quite frequently.
Basically the config bits that I like, I highlight the .rsc file in notepadd++
go to the terminal window in winbox and paste it in. voila!!!