Looking over our local rules, I couldn't find anything about a default being permanent once the defaulting team announces it in advance. But that is the way it is generally practiced in my experience. I have been on teams have been told that the other team is defaulting a line, and my captains have told those players on that line that they don't have to show up, and we would record the default.
Interestingly enough, according to our local rules regarding double-defaults:
DOUBLE FORFEITS: If both teams forfeit the same individual match (i.e. a double forfeit of the same position), the teams must reschedule the individual match and play it.
From here -http://www.norcal.usta.com/Leagues-Pages/16671_Captains_Handbook_revised_Table_of_Contents/#aMatch_Issues