Lets understand this concept with an example:
Consider that you have subscribed for the Sales edition ($12 per user) of the CRM and want to upgrade to Vtiger One edition($36 per user). Your account will be charged the difference in the price for the remaining portion of the current billing period.
If you choose to downgrade from Vtiger One edition to Sales edition, your billing process will be carried out on the next billing date.
To know more about Billing in Vtiger, click here