I made a similar change last week and was instructed to wait for an email and to call back. I received the email the next day, called and had to pay my taxes as they were refunded for my previous segment and were a different amount.

Also, I have called Singapore Airlines twice for reward bookings and they answered the phone immediately after going thru the respective prompts. Both interactions were outstanding and other airlines could learn a thing or two from Singapore. Hear me BA?