I've heard the rule of thumb on this is, if the fliud is clean (red) go ahead and change it. If it's brown and doesn't shift properly, you might be best to leave it alone.
It was explained to me that the brown is from clutch material floating around that actually helps shifting. As was mentioned before, if a bunch of crap floating around in your tranny is what makes it shift, you've got prolems somewhere down the line.
If you are planning on keeping the car and can afford a new tranny, I'd change it. You might do it good, or kill it.
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)