I'm a huge fan of Fletch and also of Jon Hamm but this movie doesn't really work. The writers have made a grave error by making most of the supporting cast quirky weirdoes with funny dialogue but this is the wrong way round - the reason the first two movies worked is that FLETCH is that guy and most everyone else is the straight man.
Here, Fletch is just kind of sleepy as he goes from scene to scene (don't look for a plot, there isn't one) being upstaged by other characters - again, literally the opposite of what makes these movies work. Hamm does have comic timing but is just too damn handsome to believe. Re-casting the role is basically impossible but while I get that Hamm would probably have looked silly in the scruffy gear that Chase wore, he's nonetheless just too stylish.
I was interested to read that the third Fletch movie was at one time going to star Jason Lee and be directed by Kevin Smith - I'd imagine that would have been a far better - and funnier - take on the character.