this thread brings up a good question to ask. if you have a questionably serviced tranny disco 1. will changing everything at once stir up crap? or just go fro it and change it all

 

If you drain the trans fluid and put 4 qts or so fluid back in most of the time you can tell if you will have problems in the future.
Now if you change all the trans fluid out at once you will find out right quick if you have a bad transmission, this would be done by a trans flush like at a service center.

 

The old "never serivce a older transmission" is a wives tale, transmissions, just like engines need service too.
You can never hurt anything buy changing the fluid and filter, if something does dislodge and float around the filter will catch it.
ATF is highly detergant, the only reason you would get sludge build up if from not servicing the transmission.