It won't damage the transmission at all. The machines I've used work off the trans pump itself, so there is no "extra" high pressure being put into the trans.
A flush is when you tap into the cooling lines and replace the old fluid with new fluid. A service is when you drop the pan, replace the filter, replace the gasket , and top it off with about five quarts of new trans oil. Although sometimes it is suggested to reuse the old gasket if it is that rubber re-useable one.
On a car with 80k, if nothing has ever been done to the trans I would suggest both a flush and service be done. But then again it depeneds on what type of driving is being done. If there is a lot of stop and go city driving then definately do both.
Cj 8)