Since you are in prepaid then you can terminate it anytime (as you have pre - paid for it already). A prepaid number is different from a postpaid number and note that you cannot retain the number you are using right now for a postpaid plan.
You can apply for a postpaid plan while you still have your prepaid.
I'm confused. Are you sure you're under "prepaid plan with contract"? I'm sure there's no such thing. But if you meant you're on prepaid subscription and loaded P300 on your account/number, then you can apply anytime for a Postpaid Plan (myLifestylePlan) via online or any Globe store. And as what Chito has pointed out, you cannot retain your prepaid number once you have the postpaid plan already. ;)