I have gotten 3 month tickets and needed to change the tickets to 6 months. I ended up paying 450 usd last month when I did this (TPE-JFK business class). When you check in at the airport, they send you to the customer service counter where you pay the money. Then, you go back and complete the check-in.
That said, when I did this last year, they forgot to ask me to pay the difference (or they just waved it).