The visor didn't move, it just looks like that:

At first I thought you had a valid point, but upon closer inspection I noticed the grey 'nose guard' (highlighted in red) didn't change place. It's still hanging above his nose, not on it.

One has to keep in mind that in the first shot we're watching from more of a top angle (to his head/helmet) compared to the second shot. This means Luke's eye seems to be behind the top of the visor (like it would if he had the visor on and was being looked at straight).

*sigh*, I'm always struggling to get my images shown....here's a link: http://www.flickr.com/photos/loconut/4931920517/

I hope this explanation isn't too hard to understand, since it's all due to spatial visualization.