Overfilling your transmission won’t damage any gaskets. The most common source of leaks would be the pan. If the pan was dropped and then put on not using proper torque specs for the pan bolts, that could be your problem. The mating surface of the pan will be bent if the bolts are put on to tightly and will leak. Other than that I can’t think of any way a leak could be caused by a trans fluid change. Go to a different shop and get a second opinion.