Originally Posted by chronicrelief

I've been doing this for a year now and yes you can essentially get all premier benefits while accruing to OZ with the following strategy:

Check in with your UA FFP on your boarding pass. Print out a physical boarding pass at a kiosk that shows UA*G. Check your bags with your increased allowance + priority handling. After check-in, you can ask the check-in agent to change your FFP to OZ. At this point your boarding group will change on all newer boarding passes but your seat assignment won't change. You will also retain your position on the upgrade list. If you want to visit the lounge, you can either show your old physical boarding pass with UA*G or simply tell them to manually enter your UA number. It is allowed within the lounge access policy to "show a star alliance gold card" when your boarding pass does not show *G status. When you board, you can use your old physical boarding pass to remain in group 1.

If check-in agent refuses to change FFP or threatens to remove your premier access, you can also change your FFP by calling in to the MileagePlus desk or at the gate. I've done this a dozen times and I've never not been able to change my FFP through one of these methods.

Note that once you change your FFP to OZ, you will not be able to select a different E+ seat without paying a fee.