I checked some TSBs and Toyota no longer has any check of the "closed" trannys untii 100k.
I can tell you that there was a significant difference in the smell and color between the WS I was draining and putting in. The kicker is that you can only do about 3.5qts at a time. Crunched the numbers and you have to drain, refill, run, drain, refill 10 times (35 qts new WS) to get the mixture down to below 10% "crappy old stuff".
Bob