As far as I know, you have to pre-terminate your existing contract to be able to upgrade your phone. And this pre-termination is subject to a fee.
Please contact 02-7301000 or 211 for more details.
There is no way for you to recontract just to upgrade your phone. You really need to finish your contract first before you can get a loyalty program. Pre-terminating your existing contract and paying the termination fee is like buying a new phone.