Recent screenshots have emerged that show what looks like a drastic change in Killzone 2. You can easily see that the models are smoother and the room is darker.
Seb Downie, QA Manager for Guerrilla Games, said on the official PlayStation forums that, "They are only the tiniest bit touched up. Short answer is yes. Long answer is that there was a little bit of colour-correction done and some minor polish, but nothing major. Still very close to reality and it looks better in motion in my opinion."
Killzone 2 is the third installment in the Killzone series that originally began on the PlayStation 2. It was first announced at E³ 2005 in Los Angeles, California, and is set for release in 2008. A lot of information is still unknown at this point.
Source: PlayStation Forum