If the transmission fluid has been changed constantly on a regular basis to keep the fluid clean and the transmission filter clean then it's safe to change the transmission fluid on the high mileage vehicle.
However if you've never changed the automatic transmission fluid or filter on the transmission then changing the transmission fluid on a high mileage vehicle that is working fine can cause more harm than good.
In fact some auto shops and transmission shops will not change the transmission fluid on a high mileage vehicle especially if the transmission fluid has never been changed.
Under normal use the transmission fluid should be every changed every 30,000 to 60,000 miles and if you do a lot of towing with a truck then the transmission fluid might need changed every 15,000 to 20,000 miles.
But I would not recommend changing the transmission fluid on a high mileage vehicle unless it's really dirty but if the fluid is fine then it shouldn't be a problem to leave the old transmission fluid in as sometimes it can cause transmission damage.