Originally Posted by FairladyZ40th

I've been told many times not to ever swap the AT fluid in a car and the differential fluid IFF, they have high miles because of the muk and gum that builds up within the system. Basically that dirtiness that builds up in there helps fill in gaps and wear within the systems and so when you put in new fluid all that helpful gum comes out and thus leaving your stuff vulnerable... My $0.02.

However if you swap your fluid early enough and continue this practice you should be good.