I'm not a pro, but I've been into a few auto trannys. I think the most ambitious job I did was to swap the guts from a big block Ford C6 into a small block C6 case (I had a 302 in a F100 that died and I bought the big block trans at the junkyard for something like $40).
Once you're in there, it's no big deal, but it does take a lot of time. Everything has to stay in order and you have to keep it very clean. If there's a problem, sometimes you can spot it easily (broken o-ring, burned clutch plate, etc. Other times though, it could be a blocked passage in the valve body, a leaking o-ring, etc. The risk in doing it yourself is that you'll take it all apart and won't find anything. Then what do you do? Put it all back and you've wasted your time? Bring it to a trans shop and confess that you've been into it (now you're REALLY gonna pay).
Not to mention that you need some special tools to take out the gearsets.
I think shops face these same dilemmas and that's why you don't see a lot of auto trannys being repaired. They just completely rebuild it, and then warrantee the repair, or they don't want to work on it at all.
What are the other perspectives out there?
Getting back to the original post - I would change the fluid.
Experience is something you don't get until just after you need it.