So you can book E+ for any future flight for free, so long as you are gold (or higher) at time you make the seat assignment. If you lose that status, and become a silver (or GM), the seats will generally stick, assuming nothing changes with regards to the itinerary. However, anything causing a change to the itinerary if you no longer hold the status - aircraft change, schedule change, you make a voluntary change, etc., - and you will likely be re-assigned at that time into E- seats.
Of course, that's how it's been in the past, and while there's no indication it will change, it always could. It's certainly possible for UA to run a sweep beginning in say, early Feb., when status shifts happen, and move anyone out of E+ who hasn't paid for it or doesn't have the status to get it for free at that time (benefits are always based on status at time of flight). UA certainly has software that runs more complex tasks than that.