I don't know that there are any guarantees about A vs C. It's possible that this may become an illegal connection on the day of travel if DCA-EWR is assigned to an A gate arrival. I suppose, at that point, you could get a positive-space rerouting by calling, assuming you can find an open seat. (If nothing else, it's worth checking the flight status on this, because if it shows an A gate, you should be able to leverage that to get confirmed on the earlier DCA-EWR flight instead of standing by for it).