On Destructoid you can find the proof that IGN indeed modified its Assassin's Creed review.
The current review contains this:
"These big open worlds, which are fully interactive, do come at a cost. There is considerable texture pop-in and occasional framerate hiccups. None of this deters from the gameplay though, so it is forgivable."
The original review contained this:
"These big open worlds, which are fully interactive, do come at a severe cost on PS3. There is considerable texture pop-in and noticeable framerate issues. Playing back-to-back with the 360 version, it's obvious that Ubisoft did not devote enough resources to the PS3 edition. The framerate is considerably worse, so much so that it begins to affect gameplay in the later levels. You can get through the first two-thirds of Assassin with the framerate being just an annoyance, but it becomes more of an issue for the final third of the missions."
IGN has changed back the review and given an explanation to Desctructoid. See the third entry at the alternative sources below.