Originally Posted by Lux Flyer

1) Departure of the flight. Check in cut-off time to be safe
2) Assuming this was ticketed directly through United you can use piece-by-piece and will have a new credit issued for any difference. If it was issued by a traditional travel agency, those credits aren't generally eligible for residual credits and you need to use the full value or lose it
3) Would need to check the fare rules for penalties. US originating itineraries have no change fees. I think most European origins don't either, but the fare rules will be the definitive answer.