When you change the billing cycle, you will be credited for the current billing period and charged for the new billing period.
On doing the change, a credit is generated overnight for the unused portion of the old billing period. Then a charge for the new billing period is posted. The charge amount will be like ordering a new site.
Any credit on the account would then be applied, with the difference being charged to the credit card on the account.
For instructions on how to switch your billing period, see This Knowledge Base article.
* If the credit card charge fails, the account will go into past due status, and we will attempt to collect again on the normal past-due schedule.