It's a golden rule in television that once you replace one of your leads in a series (especially a sitcom), your show is doomed. There are tonnes of examples of this: Spin City, Scrubs, Eight Simple Rules, to name a few.
I never liked the show, to be honest. I never found it funny even in the slightest. Having Ashton Kutcher in the show might make it at least a bit whackier (whether or not that's a good thing is up to opinion, I guess). Charlie Sheen was the WORST thing about that show - he was simply wooden and unfunny.
My prediction: the show will be canned very shortly, regardless of who they get.