The front transmission seal is a round seal with metal on the outer diameter and rubber on the inner diameter. It is driven into the front of the transmission behind the torque converter, metal to metal as an interference fit. The rubber seals the transmission’s rotating output shaft. Its name is Fred. Get the picture?
Replacement requires pulling the tranny away from the engine. It’s the only way to get to it. Unfortunately it is labor intensive.
How much is it leaking? A quart a month, week, or day? If the first, you can buy a lot of quarts of fluid for the cost of this repair. Two drops on your garage floor will spread out to a couple of inches in diameter. Looks like a lot, but it really isn’t.
I don’t see how changing the fluid could have caused this seal to leak. A fluid change could cause the pan gasket to leak IF the pan were removed during the procedure. Was it done as a “flush and fill” with a machine pumping new fluid into the transmission’s cooling lines, or was it done the old fashioned way by removing the pan? That way only changes PART of the fluid.