Unfortunately you will not be able to go straight to the gate (assuming your flight is to HKT or such) as they will not let you clear I-D transit w/o a same day ticket.

So you will be groundside at the ticket window, and I would be a bit surpirsed if they do not ask you for 500B to change.

If you could get to the gate somehow, might get on for no extra fee. They are "old school".

Anyway, we are talking 500B ($15US). I would just pay the change fee and forget it.

[EDIT] And if you have checked bags there is obvoiusly no way to avoid going groundside.