The 2017 selection carries over to 2018. If you buy more Southwest giftcards in early January they should be automatically reimbursed, but you will still probably be able to change the selection later in January. I would review posts in the other thread from Jan/Feb 2017 because I have a vague tickle that someone did something similar last year.
I found what I was looking for here:
Changing preferred airline after $200 fee has been used
Seems like last year people were able to use airline fee credit and then switch to a different airline.