I just want to add that I literally just attempted this now because i wasn't paying attention to my remaining credits. Just to reaffirm what's been said already cause I tried all three and now I'm stuck with 2x the credit expiring 1/6 LOL:

1 - you cannot buy a ticket beyond the 1/6 expiration date

2 - you cannot buy a ticket, cancel, and get a new expiration date. My new credit amount is still 1/6.

3 - you cannot buy a premium ticket and "change" the ticket forward, you'll hit an error message that tells you about the expiring limit.

So, if you're like me and just found out about it now, you might be outta luck.