tranny is not sealed. all 5-6 different versions of ZF and GM trannies have drain and fill plugs. you just need to get the new crush washer from the dealer for the change...

bmw advertises its tranny and diff fluids as lifetime. so you don't have to change it if you don't want to. nothing wrong with not changing it... nothing wrong with changing it... i just rather err on the side of caution (and spending more money) and play safe with having fresh(er) fluid in my tranny and diffs...