Circumstances change, and changing one's opinion to reflect that isn't a bad thing. Changing one's opinion on the issue of Afghanistan issue may be particularly justified, given that it's still looking like a hot mess and Obama is doing a pretty bad job of convincing anyone why it's necessary (see my piece from a few days ago that delves into public opinion on Afghanistan).
But as Peter Wehner in Commentary outlines, Will's case is severely weakened by the downright embarassing timing of his shifts on foreign policy. "We need people who are going to resist the temptation to eagerly support war at the outset and then prematurely give up on it," writes Wehner.
I agree. It's not that Will has changed his mind. It's the way in which he has changed his mind, and the events that have corresponded with that, which weaken his hold on being one of the greatest columnists in journalism.