98 Ram SS/T : Transmission filter and ATF change

I bought my 98 Ram SS/T in 2005 with 85K on it. Transmission fluid was a little Brown but not bad. There was no junk in the pan.
I replace filter and five or six quarts of ATF +3 as specified in owner's manual. I also installed a drain plug in the pan.
Here is the question: Now at 121K transmission fluid still looks nice and Red. Should I replace filter and fluid? Just fluid? Leave it alone? What interval do you guys recommend? I don't tow and no heavy use.

I always recommend every 30-35k miles but do filter and fluid.

think about it would you change your engine oil without replacing the filter.

What interval does the owners manual recomend?

I would replace the fluid. The filter is up to you.