I'm confused. You are seemingly looking to trade from the sold out show to the non-sold out show?! Just call 54 Below and switch them. I can't imagine they will give you any issue, they may even have a waitlist of people looking to go on the 19th.
Had a similar experience trying to swap out one ticket for last night's BARE Reunion Concert for a future show. I had 2 tickets and I only needed one. They said no refund and no exchange. It's a new policy that was implemented April of this year. Prior to that I could exchange for another show even though I wasn't a club member. I was ready to eat the ticket when I spoke with the manager and she allowed for the exchange this one time. I think because it was a sell-out, it was easy to find another buyer for the ticket. Also, an unused seat hurts them and the wait staff since there's a $25 food minimum with the price of admission.
You may want to plead your case to the manager. I also got no where when I spoke with the first person I talked to on the phone. Good luck!