What you are gonna do is called plan downgrade. When in contract and you need to downgrade, you will pay for the termination fee and that depends on how long will be your remaining months until your contract is finished.
Termination fee = P550.00 + (current plan x remaining months)
For out-of-contract plans, downgrading is of no charge. And based on your question, yes the bills you paid are already counted so you only need to pay the remaining months.
You will pay a pre termination fee if your still under contract... you may call the Globe Hotline and ask them for computation