Changing tags used for billing mid-month

Hi,

We use a specific tag to identify for our EC2 instances which identifies which customer/project is using a given instance for a given month. This tag is then grouped at the end of the month for billing.

The question: If we change the tag of an instance mid-month, would the billing show (for example) 2 weeks on one tag and 2 weeks on the second tag? Or would the charges switch to show up under the second tag for the entire month?

I hope this example makes sense.

Thanks, Tim