I think Yelich provides decent defense in center with a bat that is very much above average for that position. If you move him to left he'd probably be one of the better defensive left fielders but his bat would be much less special there.
I wouldn't be broken up about a Chen and Yelich trade. I just wonder if that's the best use of our resources.
A players doesn't get extra value out of his bat by playing center instead of left or right. He's still going to be a 4+ WAR guy regardless of where he plays and be the same type of hitter.