Info is incomplete, hard to review without dates. I assume you are referring UA 391 & UA 584. UA 391 shows arriving on time/early most days unless big delay. UA584 shows all over the map. If arrival/departure is Gates 70's (term 7) should not be a problem. If say Term 6 to 8, run like hell. Don't expect flight to be held. Since 2 more flights to IAD after 10PM, I would leave as is. If you miss connection, UA will auto rebook you.
I see downside if upgrading in which case you might prefer confirmed upgrade on later flight.