Thanks everyone for the input. I called the PLT desk and after doing some math the unused portion of the ticket is worthless, since the change date and change route fees exceed the existing value.

Flying back to NYC would consume a full day of travel, and I'm not sure I can swing it.

I do need to return to SEA in March, so my last option may be to buy a RT SEA-DFW now, and then a OW SEA-DFW for travel next month.