The validity of your freebie is dependent on the time you activated it. For example, if you claimed your freebie one day after registering to GoSURF, then your freebie will end on the day after your GoSURF's expiry. This means if you try to register to GoSURF again during the validity period of your old freebie, you won't be able to claim a new one.
You may call 808 to request to have your old freebie removed if you wish to change it. Once they stopped your old freebie, you can register to a new freebie.