From what I've heard, they are "killing off" Charlie's character and Ashton is going to be playing a new character. So not really replacing him in that sense. Just taking over as the lead character. As someone who really enjoys the show, I'm not so sure how successful Ashton will be. It really is a shame that Charlie had to be such jackass.
If I were the one writing the replacement episode, I'd kill of Charlie too. And just to piss off Sheen, I'd do it in two parts, with the first part as a "clip show" showing him at his worst. Anyway, he's doing a married woman. What neither of them know, is her husband is coming home early from Afghanistan and the surprise homecoming is being covered by the local news. Half blitzed, instinct kicks in and he scrambles out of the bedroom window. Had he only been sober enough to realize their apartment was on the seventh floor. The local news covers it as a related story as Jake and Alan watch television.
Part two is the funeral. Alan has a mixture of sadness and happiness. Not poor anymore. Beach house is his! Drives to the funeral in "his" Mercedes. Which brings us to the end of episode two and the reading of the will, revealing the long lost son of Charlie. Enter Kutcher...
New show dynamic. It was always Charlie/Alan with Jake on the side. Now, its Kutcher/Jake and Alan being the fussy overseer. If their lucky, they get one solid season out of it and then one with miserable, sliding ratings and cancellation.