you could have jamed something up inside the valve body of the trans itself. I had an older T-bird "Elan" (Yuck!) with an auto trans. I down shifted from overdrive into 1st/low while going something like 60 on the freeway and doing hard braking. It downshifted nice and hard and even locked up the tires a bit.
But.... when I went to accelerate again and attempted to go faster than 3rd gear (or whatever i was in) it just wouln't up-shift. Well, when in doubt, I just used to gas the car. ( I know, it's stupid ). However, I gunned it and it finally upshifted at like super-high rpm's. From then on, the car continued to drive like normal.
I'm not saying that'll work for you too, but, it's less labor intensive than changing the valve body. However, that's pretty easy compared to swapping the entire trans.