If I use 150,000miles to book CX fist from SFO to hkg using available dates, can I later pay a change fee to change the dates without having to pay any extra miles if indeed the new redemption to Asia costs more?

I'm guessing that it's safe to assume the the redmption rate on CX will increase on nov16 right? I'm just worried that an changes after nov16 will require me to use more miles.