Does changing difficulty mid-game effect trophies?

[LeoGoesGir]

I just beat Baal and started my Nightmare playthrough but logging in for the second time it's asking me what difficulty I want to play on.

 

If I switch back to normal so I can go to the cow level will that reset my progress for beating the game on Nightmare?

 

I've seen some things that make it seem like you only need to kill Baal in Nightmare for it to count for the trophy, but I can't find a confirmation of this.

[NathanielJohn]

You can switch between the different difficulty settings as many times as you like without ever losing any progress on them.

 

Basically the game keeps track of 3 levels of progress for each of your characters: your Normal progress, your Nightmare progress, and your Hell progress. Switching your difficulty setting switches between the 3 progresses, but doesn't make it forget about the other 2.