This is a great concept to pull the switch, but very hard to execute. What happens is that you are following this guy for so long, you feel really invested into his future. Then we end of following the younger guy and I was left a bit bummed out. But perhaps that's just me.
I am a tad biased when it comes to Mr. Demi Moore. I've never met the man personally, so I can't really judge him, but he's just not a leading man and The Guardian proves that. Kutcher seems to suffer from light-weight-itis. He's not someone men want to be. And I think women would like to sleep with him, but I don't think they want him to be his boyfriend for very long. He seems to lack that mixture of toughness and tenderness that countless leading men have had in the past. And Kevin Costner has it, even at 50 years old.
So when the main character switch happens, we're left with this shell of a man. And I only mean on the big screen. Kutcher is a fine actor when he's placed in the right role (think of his goofy performance on That 70s Show), but he can't carry a movie no matter what. Since THE GUARDIAN came out, Hollywood has given Kutcher countless chances to be that leading guy. But he's failed every single time. He's cursed with having leading man looks but looks are obviously not everything.
But it's hard for a guy like Kutcher to be a supporting character who's that good looking. It's like having the best player on your basketball team not be the team's captain. Because he's the best, he generally needs to be the leader. And when he's not, then the team suffers from it because it now lacks an identity. And that's what Kutcher's films are like - they all seem to lack an identity. Watch all of his past films and you'll see that I'm right on this. He's got light-weight-itis! But THE GUARDIAN could of been a contender if they went back and did some re-edits. Make sure Costner is the main character throughout and cut about 30 minutes of unneeded footage from the film. Less is more and THE GUARDIAN is a great lesson in that.