Wont void warranty by any means. This isnt "Flashing/Tuning/Messing" with the engine at all. Its simply removing all of the computers stored learning of your previous driving habits. For example if you have the 7AT and shift in manual mode at 2500 everytime, then reset the ECU and shift at 4000 everytime, throw it back into "D", and the car will shift automatically for you at 4000rpm because thats what the computer thinks you want to shift at.
At least, thats my take on it all. The harder you drive after ECU reset, the more the car comes alive. Just my opinion. It could all be in my head, but who cares.
RESET ECU alternative method