If no one knows for sure then it would be relatively straight forward to try out a replacement prs-950 screen with your prs-900. You would need to take the back off your prs-900. The screen connector is then visible and you can unplug the existing screen, lay the replacement screen alongside and plug it in for testing.
It's not going to be possible to get the old screen out of a prs-950 in one piece, since they glue them in. So buying a replacement screen is the only option.
The part number is LB071WS1-RD02 (as opposed to LB071WS1-RD01 for the prs-900 screen). I've seen it advertised as a replacement for both the prs-900 and the prs-950
e.g. link, so there is a chance it does work. Either that or people selling them are mistaken.
It's probably worth mentioning that the prs-900 has a clear plastic resisitve touch layer over the screen, and that's part of the reason the prs-950's screen looks clearer. So you might not get that much improvement. The 950 has infrared sensors around the screen instead, so you're looking at the eInk display with nothing over it.
Here's a couple of videos of people replacing the screens of other Sony readers, to give you an idea of what's involved. They both test the new screen as a first step.
https://www.youtube.com/watch?v=mdyqXS_KU34
https://www.youtube.com/watch?v=LqbYWHJk8Tk