2016 RAM 2500 complete weight loss Cummins 6.7L.

I got a new tune with EFi Live with Autocal from a new tuner.

The instructions say to flash it back to stock, then Flash the updated tune.

I'm hesitant because it will throw codes to high heaven if I flash it back to stock.

Do I flash it to stock, never start the truck, then load the new tune?

Any advice will help until I can get the tuner on the phone tomorrow/