The short-lived but much loved series Dead Like Me went on for 2 seasons and was later made into a movie, Dead Like Me: Life After Death.
All of the actors were the same as in the TV series, with the exception of Daisy.
Despite being a popular character, the TV series' actress Laura Harris was replaced by Sarah Wynter, who had played Harris' sister on a show called "24".
Why didn't Harris play Daisy?
In 2007, when the Dead Like Me movie was filming, Laura Harris had a full time job with Women's Murder Club. Apparently they couldn't work out a compromise that would let her appear in both.
You can see this implicitly on her IMDB page, but if you want an explicit statement, Yahoo Answers.
Mandy Patinkin presumably had a similar problem with Criminal Minds, but they knew about that beforehand and wrote him out of the script.
The big problem with Wynter was that she simply wasn't the same kind of actress. That might have been fine for a first time casting, but in the kind of movie that this was, it really needed Daisy to be Daisy with the Daisy/Mason chemistry. And they didn't have it.
If they'd waited, both Harris and Patinkin would have been available, but then they might have lost Callum Blue or one of the Lasses. Or perhaps Christine Willes. Or the director.
