You can never be sure about how the post office works. They forwarded government correspondents for many people I know. So it never hurts to do the forwarding.
On the other hand, you are right that they are not supposed to forward mail. So the best would be:
1. send in AR11
2. do the forwarding
3. call immigration if you have their number to change your address. The problem is it is difficult to get them on the phone.
4. watch the on-line case status and if there is a change, look for mail and if the mail does not arrive, call the 800 number to double check.