It's not surprising that 'Two and a Half Men' is set to get a replacement man -- as you've no doubt heard, Ashton Kutcher will join the show next season.

It will be surprising, however, if the substitution actually works.

It's not just that Kutcher has a very different presence and style than Charlie Sheen. I'm certainly not defending Sheen, whose departure from 'Men' was necessary and appropriate, but you can't argue with the fact that his wayward character had a dark cynicism that grounded 'Men's' lightweight comedy in a very specific way.

Sheen could be both a straight man and the comic centerpiece, while Kutcher seemed most comfortable as a goofball supporting character during his tenure on 'That '70s Show' (don't ask me about the roles Kutcher has played in his film career -- you couldn't get me to watch his movies for love or money).

Could Kutcher work on 'Men'? I suppose it's possible, but the show will have to retool extensively to integrate his very different style and personality, and I'm guessing the fans of the show don't want the show to change.

And let's face it, history tells us that when key actors are replaced on a TV series with limited core casts, shows usually never get better. In fact, they often get worse.

Nobody's disputing that 'Bewitched' was pretty much the same show no matter which actor was playing Darrin, and this list of sitcom departures and substitutions confirms that when subsidiary characters are replaced, it's often not that big a deal. But some high-profile programs who've lost integral, if not defining, personalities haven't fared that well. A few examples:

'American Idol': The show lost some unpredictability when Paula Abdul left, but it lost its spine when Simon Cowell left. The current crop of judges overpraise every mediocre performance so much that it feels like we're backstage at a grade-school music recital ("I love you!" "You're amazing!" "You can do anything!").

'The X-Files': Fox should have killed off 'The X-Files' when David Duchovny left, but noooo, the network just couldn't resist milking that sci-fi cash cow for a few more years -- and thus providing the ultimate example of a television show that destroyed its own excellent reputation and limped into retirement as a shadow of its former self.

'CSI': Does anyone out there think that the show got better after William Petersen left? It took the crime drama a long time to find the right tone and arc for Laurence Fishburne's character, who doesn't have the same quietly commanding vibe that Gil Grissom did.

How integral Grissom was to that drama's ensemble became even more apparent after Peterson left the show, but high-profile departures like that are usually more unsettling for shows with smaller casts. If a show has a small group of central characters, losing one member of the cast can throw the program off balance for the remainder of its life.

'M*A*S*H,' 'Law & Order' and 'ER' lost quite a few cast members over the years but did all right after those departures, mainly because those shows are ensemble-based, not centered on two or three people. Adding newcomers to those programs wasn't that big a deal, for the same reason that 'NYPD Blue' was able to absorb the loss of David Caruso well: There were other characters that the show could focus on, which meant it never really lost its way.

But 'The X-Files' was about Mulder and Scully, really, just as 'Two and a Half Men' is about the three men at the core of the show (Angus T. Jones hasn't been "half a man" for a few years now). Losing one of those cast members and throwing in an actor with a completely different vibe sounds like a recipe for failure. But maybe all the fans of the CBS comedy really want are the sexual innuendos and double entendres. Who knows, as long as those shopworn, bawdy jokes keep flowing, the show might do all right.

My guess is, continuing 'Men' for even one more season is going to be so lucrative for CBS and Warner Bros. TV, the studio that makes the show, that the quality (or lack thereof) won't be an issue. Maybe we'll only get one Ashton season, which will function as an asterisk in 'Men's' history, but the bean counters will be pleased, which may well be the point of the switcheroo.

Do you disagree? Do you think the Ashton-Charlie replacement will work? Or do you think 'Men' should have just been canceled, as I argued months ago? Sound off in comments.

Follow @MoRyan on Twitter.