Good news from my experience: I had the same thing happen to me in February - - a TPA - MEM - MSP connection was changed from two separate flights to the same TPA-MSP flight number - - and in fact it was the same plane arriving and leaving from the same gate in MEM.

I made sure to get off the plane in MEM (got BBQ of course) and re-scan the boarding pass for the MEM-MSP portion, and got separate mileages credits for each leg, with credit for two segments. ^