Depends. Your monthly bill gets pro-rated to how many days of the cycle you’re in. That’s the easy part. The depends part is any outstanding tab balance. You’ll have to pay that off plus the tab subsidy portion. That’s the part that could be a lot, depending on the phone on tab, if any.

You will be charged a pro-rated amount from the start of your billing cycle to the date you switch to the prepaid plan. 

If you would go over your prorated plan amount, instead of charging you overage I Thi k koood bills you for the full month.