I'm going to venture a guess that the USH segments are treated just like any other air segment, as they are CO-codeshares. For example, TSS-EWR-LAX is not really any different from DCA-EWR-LAX.
So if you want to make any type of change, and you're on a nonrefundable fare, then you're going to have to pay the change fee and re-issue the ticket.