Actually, I already provide this in the release notes. A PROTOCOL_VERSION bump means restarting tmux. We didn't do this between 1.9a -> 2.0, hence why it wasn't mentioned. Look at the release notes for 1.9, you'll see it was mentioned there. Simples.
Is this a session-breaking update?
I'd hate to run the update only to have it kill my current TMUX session.
Devs - if you're reading this: PLEASE indicate if I could update without restarting the session.