Officially, Stewart has a leg injury and that is the reason for the change. Stewart said if he was healthy he would be the starter this week, but Jauron danced around that when he was asked if that was the case. Basically, they wanted to make a change and so they can say that Stewart is "injured" which is technically true (isn't pretty much everybody in the NFL technically injured by wek 7) and if Chandler does well this week they can go with him and if he doesn't they go back to Stewary while still saving face.
However, I think the Bears should already have a hospital room reserved in Seattle in preparation for the 12 times Chandler's going to get sacked Sunday night. Some QBs try to win the game, some QBs just try to keep it close, Chandler will just be trying not to get decapitated.
This is one of the only games where I cannot even come up with a far-fetched, half-ass scenario where the Bears stay close in this one.