I'm just going by bits and pieces of various articles, but I believe in Florida the case can be dismissed immedately by the judge as not being a prima facie case (not enough evidence to even respond to), which is not same as a mistrial, and a retrial would not be an option.
Now changing to a reduced charge like negligent homicide - I don't know if that is an option at this point. If it is, and I was GZ, that is one that would concern me a lot.
Drop George Zimmermans murder charge by The Dersh