Could have harm if they use the cooler lines and damage them but new fluid is typically better than old fluid IMO
There are dipstick flush machines too. If you have an extractor you can pull your fluid maybe 2-3 quarts from the tube then top off with new atf+4 and do that every few oil changes if you like
Biggest thing on these 62tes of that vintage is the torque converter failing, clogging the filter starving the pump
There are some random hard failures like compounders and occasional input clutch area, you get the occasional solenoid on the valve body the variable force one that’s for the converter lock up start to go bad, or valve body solenoid switch valve bore wear, but mostly it’s not a bad trans until the converter goes