Overfull the transmission fluid may foam. If this occurs the control circuits do not see the right pressure I suspect. All I really know is I have found these transmissions to be pretty sensitive to fluid level.

As a general rule if the old transmission fluid did not look really bad changing it the way you did should seldom cause issues. You could not pay me to power flush one of these old transmissions though.

What is concerning at the same time is you mentioning the transmission seemed normal for the first day after the change.This indicates that you may have a real problem.

The transmission fluids have been upgraded over the years and a fresh higher level of detergents in the newer fluids may have had some effect. I think tractor dealers still sell the old types of lower detergent transmission fluid.

I do not know much about automatic transmissions but suspect the newer detergents may have gotten some junk loosened up in that box or somehow caused an internal seal issue.

I might add a bottle of transmission snake oil just in case some internal seals may have been compromised. It will swell them back up but takes some miles to do this. Or at least would try this if it becomes evident the transmission has to come out otherwise. The transmission has a top vent and if the overfilled fluid foamed some may have been expelled through it accounting for your puddle.

On the other hand the wife has not had enough time to correct my thoughts on your issue. So hopefully I am just wrong as usual.