Pricing Switch to Reservation

We're currently on a Fabric pay-as-you-go model which is billed following the month of service.  We'd like to switch to reservation pricing which looks like it is billed immediately for the upcoming month.  Based on this, I don't see a way to not be billed twice in the same month as we make this switch.  Am I seeing this correctly that pay-as-you-go is payment after the month, whereas reservation is pay upfront?  Is there a way to time this switch so that we're not billed twice for the service in the same month?