... A better option would be for UA to accept future seat assignments and if/when subscription renewal doesn't happen, to run a simple program that un-assigns those seats.
Next post, UA allowed me to select E+ and then kicked me out
Allowing one to extend present subscription with a early renewal but that locks the price for perhaps too long in UA's mind.
Another approach is to replace and refund the old subscription upon renewing -- limiting the subscription to 1 year in the future , maybe 13 months to allow the present 30 day window. But this is getting too complex and prone to problems.