Upgrades and downgrades that are done through the Admin Panel effectuated immediately.
On downgrades, credits will be issued for the period from the time of upgrade and until the end of the billing period. These credits are automatically recorded to your account, and will automatically be used for payment in the first coming future transaction. Credits are non-refundable and non-transferrable, but can be used for payments for any Servebolt service.
Upgrades require an extra payment for the period from the time of upgrade until the end of the billing period. If there are credits on the account from earlier downgrades, these will be used first.
Specifications of what happens with regards to financial transactions during up- and downgrades is clearly visible during the checkout process. If you want to know more about how our billing works, we have made an article for just that.