Thank you all for your helping out. Yes, it is possible and we just got it done. I called UA and explained what I want to do, and she said it is possible if R space is still available. And for the flights in question, R spaces are indeed still aplenty. So I called back UA, and conference called in my friend for a 3-way conference call, and UA is able to redeposit the GPUs to my friend's account, then apply my GPUs to the flights.