When mine packed up, it would slip in 2nd and 3rd - seemed like it wasn't changing up, but what I think happens is the clutches between 2nd and 3rd wear out. Pulling away (in first) would be Ok, but after that it was pot luck how much drive you would get with the engine revs not really relating to speed. When it changed into 4th things were Ok again, but there would be a noticible clunk/thud as 4th gear engaged. Things were worse on a cold engine/gearbox and generally Ok on a warm engine/gearbox and for a while I convinced myself that it was just the gearbox ECU holding onto revs whilst the oil warmed up. There may have been a bit of that, but not really the cause.
I think by the time you notice things are going wrong the damage has been done, and no amount of changing fluid or filters will fix it.
There are a few videos on Youtube of people reconditioning them. I think the US equivalent is something like GM4L60. Doesn't look too difficult, but you've got to have the time and be able to get the spare parts. So easier to buy a spare if you can find one.