I've just installed the Breaking Ground expansion and I get an error message saying "Breaking Ground isn't a recognised expansion that can be loaded"

I'm playing on Mac with KSP version 1.6.1, I followed the installation instructions and installed it into the applications folder- is it meant to be installed directly into the KSP folder?

Okay problem solved, in the installer you have to instal it to the Kerbal Space Program root file. Also have to have updated to 1.7.1... bye bye old saves!

You should be able to keep old versions of the game around (assuming you have the ~4GB drive space to spare), and run them independantly. The game's DRM-free from all stores, not just gog.

If you've already updated but still have your old saves around, and you're using the Galaxy client, it does have the ability to roll back as far as v1.6.1. However, brand new saves are definitely not going to work if you do that.

HTH