Upgrading from 2.1.5 to 4.0

We’re in the same position i.t.o server versions. We’re currently on server v2.5.1, using v1.3.3 of the c# client. I’m assuming we’ll need to upgrade to some newer version of the c# sdk in order for it operate correctly, say v2.2.8.

Will the methods we’re currently using still work, or will we be looking at rewriting some of this?

Thanks!
Jared