First step, I set up a proxy, to avoid many repetive downloads, or at least to just download them from my ssd's.
I've used the proxy.sh as described here: Shell proxy autoconfiguration and just manually filled the CFEngine variables.
(As I'm writing this I feel like an idiot for not letting Rudder do that)
Then we sed all 42.1 repos to 42.2 and remove, if it exists, the cd repo from the original install.
After that a refresh works just fine and we can run the update.
This is pretty much all you need!