As long as the fluid does not smell really burnt, then another option would be to collect the fluid, then when you remove the filter check the pans magnet, it should have the typical greasy goo on it, if you see big metal flakes then consider putting the old fluid back in after the filter change.
Also like I suggested use a rare earth magnet, and check over the fluid well with it, if you see no big metal flakes, then go ahead and put a new filter on, close it back up, and top it off with Dexron 6, you'll still have a bit of the old fluid in the system, so you won't be going with all new fluid...
Whatever you do just don't get a full flush, unless you have the money to rebuilt the transmission in case it cause it to fail.
Either way eventually failure is going to happen, I had slipping issues with my tranny, and a filter change, then topping it off actually helped considerably, it all depends on the state of the clutches.
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA