Joanek - Stop worrying and looking on ITN for upgradeable seats on that run. There is no discernable pattern that I have been able to find over the years to gauge whether the FC cabin will be full between DCA and EWR.
For example, as Viajero Joven suggests, one would think that Friday nights would be busy and that is almost always the case, but one would also think that Saturday PMs would not, and that frequently is not the case as international BF travelers sometimes take up the entire FC cabin. I will bet you a cocktail at the PC (I'm a big sport) that you do get offered the upgrade despite what it looks like a few days in advance. Let us know how you make out on the trip (but not at the ballgame, because we know how bad the Sawx are going to be the way things are going for them this spring).