As long as yours is misbehaving, getting new fluid certainly might help. Also, I've heard SeaFoam TransTune can help with problems on old transmissions. It might be a good last ditch effort.
http://www.seafoamsales.com/transTune.htm My 96 automatic has 127,400 miles (original transmission) and it seems to still shift good. Bentley lays out a tranny replacement pretty well, but... I think the hardest part about an automatic replacement is getting the correct fluid level. The level has to be checked with the tranny at a specific temperature range.
www.pelicanparts.com has a good article about auto fluid changes.
Would you look for a fresh rebuild or a used salvage yard unit?