Hey David,
The stats should be cumulative. Every time the campaign is run, it will consolidate the send. If you ever wanted to A/B test, all you would need to do is clone the existing email, make your changes, and rename it to signify the difference. Once that is sent, the email stats within the Program will show two lines; one for each email.
I have not done this type of sending in a while, but I am pretty sure that the stats are aggregated after each send. You can always test!