It is a great point
I am surprised I didn't think of that either. Our plan today in booking will be to book a flight MIA-IAD on the day later, that is available. If the flight we want opens up then we'll switch to that. Otherwise, as it gets closer we'll just change it to a return in, say, February, then buy a one-way down to MIA and use the return that way.
Bags - yes not a problem - will be claimed in MIA, if checked at all.
Thanks for the input! I just want to be relatively sure that we won't get socked for the $100 change fee to miss the flght.
Now of course, how to decide when/if to buy the $90 one-way from MIA - IAD is another story...