YMMV but I know myself last year, (I switched to DL because of destinations, but have picked up UA status this year again) and a few co-workers this year got upgrades on tickets bought before going to zero status. I was told by a UA rep that the status gets marked on the ticket when you buy it, or if it needs re-ticketing. She pointed out though that this means no stand-by and your scheduled flights better make it, otherwise the re-ticketing will clear the status.
However I know officially that you should not be entitled to benefits,thus YMMV.