The fear is that flushing or even adding new fluid could dislodge some crud that would end up clogging up an orifice. New fluid has a detergent-like quality which may take a while to loosen up any crud (if any exists). Therein lies the risk with just a fluid change.

If the current fluid is pretty cruddy that will probably increase your risk of dislodging other crud from maintenance activities. If its clean, your odds are better. I dunno what to recommend - new fluid would help extend the life of the tranny but with risk. Perhaps your best option would be to throw a big tranny cooler on it. That will help extend tranny life and be virtually risk-free for dislodging crud.