Yep, what Art said. One of the last few models that still used good ole Dextron ll. Start getting to models newer than that they start wanting Dextron lll and synthetics and all that ********.
Hopefully that hard downshift was just a fluke and doesn't continue. I'm doubtful that had anything to do with fluid level if you were just on the add line. When 700r4 start having hard upshifts and/or downshifts, it's a sign the end is coming near. Maybe not in the next few hundred miles but it's time to start babying it a bit. If it was a one time thing, I wouldn't worry about it, carry on. it could have been a sticky valve that freed itself up and all is now well. If it become more frequent, then I'd be more concerned.