Powering the UNO R3

I'm powering my UNO project with a 6v battery pack through Vin and Neg. My question is; if the battery pack started to go low and I then plugged in a 9v DC wall transformer through the 7-12V input jack, leaving the battery pack still connected, would it hurt or fry the UNO ?

Maybe not the Arduino, but it would connect the 9V plugpack to the 6V battery which probably wouldn't do it any good.
6V is also too low to power the Arduino via VIN, you should be using at least 7V.