I would not want to go over 50K without a fluid change. After 200K on original fluid - forget it.
There are two basic types of transmission flush machines - one that uses its own pump to power the flush and the other type uses the trans' own pump to move the fluid. The latter is perfectly safe for any modern transmission, but I have never had an issue with the former type, either. As for just dropping the pan, if all you want to do is change about 1/4 of the total ATF in the system that's fine, but I prefer to replace it all with fresh fluid.