I dont think there was any conclusive evidence to suggest that changing an old tranny fluid (i.e. if its brown) does any damage to the tranny in whatever manner consistent with what we've read here.
I think its more of a myth than anything else, as I assume most mechanics have good intentions on their customers.
It is my considered view that it is fine to change it, and ought to be done according to the recommendations.
If we're afraid of future leakages due to an old tranny (which I do not subscribe to), then it may be wise to pour in some additives, e.g. Gear-Up.
126 tailed by a 203, 129 leading the pack.