Since this thread has popped up, I'll post my recent unexpected experience with this.
I booked in December for flights Feb 3 & 4 on UA. I was Silver at time of booking. I flew very little on UA last year and knew I'd expire to general member before flying. I expired down to "member" on Feb 1.
At check-in on Feb 2, my account showed "Member", but my boarding pass showed "premier access" and "Premier Silver". I checked in to my original E- seat and then decided to pay for an E+ seat, since the outbound was a redeye. On changing seats it showed me prices for the seats. After changing seats, there was no confirm charges page, and my BP continued to show "Premier Silver". I refrained from posting in case UA reads this and suddenly gets better at IT (unlikely, I'm sure...). I flew the leg in my E+ seat.
At check in on Feb 3 for my Feb 4 flight, it *still* showed me as silver on the BP. Only E+ middles were available at the time, and I had an aisle so I didn't change. A few hours later an E+ aisle showed up, I changed into it. It showed a price on the seat, but didn't charge me. My BP continued to show "Premier Silver" while my account shows "member".
I think this isn't what they intend, but I'm not going to complain...