Buh... unless I'm being stupid, everything both of you said is right.
Anyway:
- The server is now upgraded to 3.26. Hooray! This is an excellent patch for modding.
- You can send turns in using either version. You will get no problems either way.
- When you get new turns back from the server, you will only be able to open them with 3.26. If you try to open them using 3.24 then dominions will tell you to upgrade (like Executor said, I think).
- Basically it is impossible to do anything wrong.