Good Day @ @pmg008 ,
All unused portions of the phone and internet service will be credited when you change the plan.
I recommend having a read of the following TPG Community articles below which will guide you on how to access and check your billing info online via My Account:
Your billing cycle and advance payments explained
TPG My Account - Accessing your account information online
Please check it out and let me know should you need further contact for assistance.
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