If you upgrade your package, you get a credit of 70% of the price of your initial package. This allows you to make a smooth transition as your needs grow, saving you time and money (that would otherwise have been spent on learning new systems, migrating data, etc.)
For example, if you start with Silver ($5,900) and, later on, you want to upgrade to Gold ($9,900), you get a credit of $4,130 ($5,900 x 70%) towards your Gold package. So your Gold package will only cost you: $5,770 ($9,900 - $4,130).
You keep all the hours you have in the bank of hours.