Family Guy
You might not be aware that Mila Kunis voices the unloved daughter Meg from Family Guy. This role has earned her some pretty sizable paychecks over the years, but they almost went to Lacey Chabert. Lacey was pursuing a degree at the time, so she passed on the role to focus on her schoolwork. Hopefully, the degree better is worth more than Kunis' $175,000 and $225,000 per episode...
Hannibal
In a show about cannibals, how could you focus on anything else? Originally, Michael Pitt played the psychopath Mason Verger on the program. Once the character permanently altered his face, however, it went to the actor Joe Anderson. The timing just so happened to work out perfectly with recasting and shooting. The rest is history...