New client with old server files?

hey, is it possible to use the new client together with the old server files?

Impossible. The way the client interacts with the server is different. However, you can still copy Official Ressystem, by replacing all the rows and get a limited Official-Like server. Hope this helped. :)

^Nope, a middle man server could fix that.

Hmm, true that, but direct connection won't work for sure.

Isn't that how Terra has it?

For their launcher login yes.