Immediately after the change? Prolly got a particle disdodged and now blocking a passage. Hey, yer tranny's havin a stroke!

Seriously though, if this was immediately after your change, I'd work with it a bit - meaning if you can get it out and it drives ok forward, try that for a bit and see if heat/pressure changes free something up. Can't hurt. I would also consider finding a way to power-flush it perhaps........

Worst case then it comes out and apart. Comes all the way apart to get to the reverse clutch and such, but doesn't take anything fancy in the way of tools and stuff. Then again it IS January..........