what makes you think the problem is in the transmission? why did you change the fluid? if you used oil in the tranny, and drove it, you’ve probably ruined it. it takes a specific TRANSMISSION FLUID, probably Dextron/Mercon III. read your dipstick, which probably tells you how to check it too. nice thing is, if it’s a four cylinder, there are hundreds of ttransmissions at salvage yards. the trannys almost always out lasted the 4-cyl engines.
checking the fluid level in park is the standard procedure. what was the level of fluid? Don’t believe everything the kids at Autozone tell you. If it’s not in their computer, they don’t know about it.