Please excuse my ignorant here, but can someone explain to me what exactly get disturb by draining out the old tranny fluid and replacing it with new fluid? If there's any particles at all that are lodged in the tranny somewhere, the chances of it getting dislodged would be just as likely whether the fluid is new or old?

The whole "lifetime (100k)" fluid sound like a planned obsolescence to me. BMW can either sell you a $6K tranny or a new car!