It looks like they were initially trying to charge the same tax twice: 2 X $38.20 = $76.40. Maybe you would have received a credit for the first tax payment eventually (if it would be an automatic refund when the ticket is reissued) or maybe the system wasn't recognizing that you paid the tax the first time.

OTOH, if it's a voluntary change and the tax has increased since the first ticket, it doesn't seem unreasonable to be expected to pay the difference. This seems to happen on revenue tickets, for example if one SDCs and adds a connection versus an initial nonstop flight.