Yes you should downgrade back to .65 before you give it back. Unless Verizon officially releases it before that time. In order to downgrade, go to control panel>add/remove programs, and delete .75 from your computer. The d/l .65, install it to your computer>plug in device> it should prompt to update.
Forgive my ignorance but where do I re-dl the .65?
You should be able to d/l it HERE but if that doesn't work, just delete .75 off of your computer, and then select the update option on the phone or --> BlackBerry - Update your Device Software
Because having software on a phone that isn't authorized yet by the carrier is bad. It voids warrantees, and if you sent it back to them like that they probably wouldn't accept it and tell you that you screwed up the phone.