I've heard from the beginning of time that a partial change is the way to go if the tranny has high miles. I've heard a bunch of different reasons, but Richtazz put forth a theory that made the most sense to me. By replacing all the fluid, you bring the shear strength of the fluid back to new, and it can be too much for older clutch packs, and pull the lining off of them. New fluid will strip away varnishes that may have built up and are cushioning some parts as well. New fluid will generate higher pressures, which may be too much for older seals.
I can't confirm this, but I can tell you that if you want to kill a tranny that isn't acting right, changing all the fluid is the quickest way to do it.
If the service was performed at regular intervals, like 25,000 miles, I wouldn't hesitate to do a full exchange.