for 2004 BMW suggested changing the fluid at 100k, at least, in my owners manual...

I really would go to an independent shop and have them change the fluid. You may be able to get a few more miles out of it.

Nonetheless...you should probably brace for a tranny rebuild. As to the fellow who says that it requires a special shop...I don't buy that. See this thread as to why...

My Transmission story
Transmission Story part II

If a reasonably competent fellow is willing to rebuild one on his dining room table, without any fancy tools...and about $500 in parts...

Transmissions are actually pretty simple...they're not nearly as scary as people make them out to be. Just follow some really good instructions, make sure everything is very clean, and be sure to soak your new clutches in tranny fluid before you install them.

Stripped: E46 Automatic Transmission. Must see epicness, period. - E46Fanatics

When mine dies (and it's showing no signs of doing so at 70k), but I did change the fluid at 50k and will continue to do so every 50k), I'll rebuild it myself. On my dining room table :-)