I think you'd have a case if you want to go legal - the terms of your contract as I read them say it's a free of charge change, but nobody has yet pushed them on this as far as I am aware. If it's ex-HKG then the amount in dispute is likely to be less than HK$50K, so you could take it to the HKG small claims tribunal.

If you let them reissue then not only will you lose the 10% discount, you'd also have to pay the current price, which could be substantially higher again (depending on where you originate).