This article explains how the Careview subscription tiers are calculated.

When your Organisation moves up or down a subscription tier, you are prorated based on when the subscription tier changed in your monthly billing cycle. 

If you change tiers anytime through your monthly billing cycle, you would be charged at at the applicable subscription rate for the remainder of the month.  Your invoice would include an adjustment to reflect the change in subscription tiers, as well as your next monthly fee as Careview is charged a month in advance.