Systems only knows what status you are at time of booking, not at some point in the future. Even those keeping their status still see the expiry as Jan. 31, 2018 until Feb. begins.

All gold gold benefits will drop at the beginning of February, including ability to select E+. But if you’ve already selected an E+ seat, it should stay, unless there is some change to the flights, particularly plane type. Bag allowance will revert to silver benefits since travel is as a silver.

Note this is based on past (and I’m gold dropping to silver this year too, just booked a trip for mid-Feb with seats in E+). However, would probably be pretty easy for UA to sweep and remove ineligible pax from E+. They haven’t done this in the past, but no reason why they couldn’t.