Changing between Pro Packs

Hi,

I am planning on buying Pro version real soon. The pack I am looking into now is 60$ a Year one. Since I have just 8 domains now. I just wanted to enquire that what if I need to change to next plan during the year. Will I have to buy a new pack for that or can current one be upgraded by paying the balance amount. I can not go with monthly packs as I dont have my own Credit Card. I will have to borrow one from my Dad. I can pay him once a year but not monthly.

Thanks