I don't believe the ability to reset missions will be available in any mode other than UVHM. At least that's how I read it.
In that case, you wouldn't be able to grab a level 50 one, but you could grab one at, say, level 55 or whatever you'd be at when you do that mission the first time in UVHM and then, when you hit 61, reset the missions and get one at that level.
NICE! That is what I was hoping it meant! That would be perfect for my Siren.
I have Dysarthria. It's a Speech problem. Your question is answered now.
I believe you can only reset game progression in UVHM. So say you get to Sanctuary and want to start over. You can restart UVHM and you'll be back at "Cleaning out the Burg" mission which is the starting point for UVHM.
You cannot reset individual missions. Just game progress.
There's a dude in the bushes--HE'S GOT A GUN, what's he doin? I don't know I don't know, RED TEAM GO RED TEAM GO!