The reason it is so important to change the filter on an auto tranny is b/c there is constant wearing of the clutch paks and the aluminum sliding parts ..and where do you suppose these worn off debris wind up ??? .. in the filter.
The suction guys will tell you it winds up suspended in the fliud ..it does..and then the fluid just carries it to the filter.
When changing a trans fliud/filter, I open up the old filters so I can get a look at the clutch debris and aluminum wear content..this gives an insight to tranny wear ..if you ever opened one up, you would see why you want to change it..new fluid can not go through an old filter any better than the old fluid could..a blocked filter restricts oil flow and that is the last thing you want happening inside your trans..you are better off with a new filter and the old fluid than you are with new fluid trying to go through an old clogged filter...
Change them both and clean out the pan while you have it off...they can agitate all they want, they don't get the crud out of the corners of the pan like you can with it off. Do it right..trannies biggest problems are DIRT caused !!!