Paying off Tab + Number change

Hi, 

I'm on an $80/mth plan with Fido on my iPhone 6S and wanted to pay off the $400-something balance I have left so I can go to a Bring Your Phone Plan with Fido. If I go through with this, will the remaining balance be charged to my bill for next month? 

Second question, I've tried to change my number online but I can't go through. I'd like some help with that. 

Thanks

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