Transmission slipping (clunks) in reverse

I have a 2000 323Ci with automatic steptronic transmission. It has 100K miles on it and I drive 90% freeway. I'm the first owner and drive very gently. Recently and intermittenly, I've noticed that when I put it in reverse, the transmission slips a little for 1-2 seconds, then clunks (the car jerks), then fully engages. All forward gears are fine. No warning lights from the dash. No transmission oil change has been done on the car since it's supposed to be a lifetime maintanence free tranny. Is this a sign of transmission going bad? Or is this something an transmission oil change may fix?

Anyone know if there's a DYI for an automatic transmission fluid change?

Any help is greatly appreciated. Thanks!

If you are not a member of the BMW CCA please sign up/ Thry have a tech writer (Mike) who definitely says transmission oil should should be changed at 30K miles. My 330ci has a different tranny than yours and with 33K I am thinking of changing the oil. Just got to figure out who will do it. O am speaking of the Roundel Magazine.

Called 2 different BMW dealerships today and asked for pricing on ATF change. Both gals from service dept say that the automatic transmission that I have is a "Closed System" and they do not perform fluid change on them. She told me to bring in the car and test drive it with a tech to replicate the problem. I may just need to bring it in and hope I can reproduce the problem..., but on the other hand, I hope this problem doesn't occur everytime