Drop the pan and put new fluid and filter in, wash the pan out good and make sure it's 100% dry before putting it back, replace the gasket if necessary.
With this high mileage issue, it's the tranny FLUSH that you want to avoid. It uses high pressure to blast away gunk and varnish that can really destroy an older transmission, or one that hasnt been taken care of.
Imo, it's never bad to put fresh fluid in, it's responsible for lubrication and cooling just as engine oil is, and just like engine oil, it loses it's ability to lubricate and cool efficiently over time... just as it's been stated earlier in this thread, your transmission is older, might have crappy old fluid in it to begin with, stick with the Dex 3 to be safe.
If the fluid is really dark and smells burnt, probably wanna get it looked at, cause likely damage has already been done.