This is a very opinionated and controversial topic to say the least…
If you have zero history on the transmission maintenance or repairs then I would highly recommend taking it to a transmission shop and have them check it out 1st…
Changing your fluid cannot cause any damage by itself, if everything is fine inside the transmission… The issues start to appear when you change the fluid if your clutches are already worn or damaged from use. If you go and replace the fluid after they are already worn out. this can cause slipping…
Look at the color of the fluid, smell the fluid,… And worst case, drop the pan and use a transmission drain pan to catch all the fluid that comes out and then see what all is in the pan… An experienced transmission builder can tell the differences in materials in the bottom of the pan, like friction material, variety of materials, such as bronze, steel-backed babbitt, plastic, nylon, or cast iron, parts of snap-rings etc etc… You can feel the grit from the clutch friction material…
You can put the old fluid right back in it if it looks like it will require a rebuild… Done it many times when a customer didn’t want or have the budget to rebuild the transmission…
What happens is the same friction material that makes the clutch friction disc grip to the steels in the clutch packs (bands are the same way) as it wears off the friction material from the disc mixes with the ATF and causes it to grip… Remove that gritty fluid and put in fresh ATF, now you no longer have (or very little) that grit in the fluid as well as on the clutch fabrics (disc) and now it slips or no longer moves under it’s own power…
These are what old (used) good friction disc look like during a rebuild… Notice the teeth on the inner part of the disc…
These are very bad friction disc on a rebuild… to the right, notice the inner teeth disc, the friction material is about gone…
And these are wasted… lol
EDIT: Without any history, you could have a factory new transmission with low miles, but then again it could be worn out…