No danger. Make sure your reservation shows "R" class on both flights and make sure the e-ticket that goes with the itinerary shows "R" class. As long as it does you're good to go.

If the reservation hasn't been re-ticketed and you go to check-in you'll be blocked from doing so. That's when things could get interesting and not in a good day.

I'd call UA and just ask if the ticket was re-issued after you applied the GPU. That's all you need to ask.

-RM