United MileagePlus (Consolidated) - Upgrade and Flight Change on a M Bucket Fare

Hi folks - I was booked a flight by a third party on a M bucket fare. Unfortunately, the timing on the return is tight and I suspect I probably will miss the flight. I also noticed that it would only cost an additional $146 to upgrade to domestic first (I'm a premier silver and I've only been upgraded once on this routing). Does anyone know if domestic first fare buckets allow for complimentary flight changes? Thanks in advance!

You would be buying a B UP or a deeply discounted Domestic first (Z fare) so no complimentary flight changes. If you miss your connection and you have a Z Fare, they would be obliged to put you in First on the next available flight. Not so with a B UP. Missing the whole return at best would be the "Flat Tire Rule" so no love (free changes) there.