In my experience just replacing the filter and fluid (drain tc) has proven successful 100% of the time.It matters more how frequently in my opinion ,I just simply do the service once a yr on anything I daily drive.The success goes to how many trannies Ive had to lower ,which to date is zero .What does tend to put a 617 / trannie on the side yard is the b2 piston leaking at seal (no forward gears) or vacuum box (no 2nd 3rd or 4th gear shift),these are both a very doable diy project which Im in process with right now on a 300sd and another 300cd.If you like to experience no issues with these very finely built transmissions just baby them like you do your 617s.