Asuming the customer already used 2 months of hosting and payed $ 100.-
1. Agree to a price per year for the upgraded account, including discounts you want to give etc. (lets say $ 200.-
Since there is only 10 months left in the year, he only has to pay 10/12th of 200.- is $ 166.67
2. Since he already payed you $ 100.- you deduct this amount. But only 10/12th of it because he already used up 2 months.
Used up: $ 16.67, amount left 83.33
So he pays you 166.67 - 83.33 = $ 83.34 extra for the remaining 10 months.
He has now paid 2 months hosting at $ 100.- per year
10 months hosting at $ 200.- per year