Originally Posted by emcampbe

From the T + Cs on the subscription page (https://www.united.com/ual/en/us/fly...my-plus.html):


I'd guess technically, you're not supposed to be changing the FF number, as your MP number would no longer be included in the reservation. While UA may not typically filter people out of E+ seats, you certainly would be if there is itinerary changes, such as aircraft configuration change or aircraft swap, and they are probably able to do so based on the T+Cs, even after check in.

If you plan to do this, I'd do it at check in or after.