You can upgrade your account via you domain control panel upgrade button. Our system will calculate what you have left in unused $ and what it will cost for the new plan. Upgrades are instant..
The amount of traffic varies with your hosting plan. Less then 1 gig of traffic is normally plenty for a normal web site. Our Basic hosting plan comes with 6 GIG. The transfer rate is calculated on a monthly basis starting at the 1st of the month.