There was no change to SYG last year, as SYG only refers to not having a duty to retreat. That's been the same in Florida for years, as it is in most states.
What changed last year was related to self defense immunity. That had to do with the self defense immunity hearing. Florida has a law that prohibits you from being arrested or tried for exercising your right to self defense. That's why the shooter in this case has not been arrested yet. But if the cops don't agree that it's self defense then at some point the system has to make that determination. So to avoid trial a defendant can request a self defense immunity hearing.
In that hearing the state has to prove that it was not self defense by "clear and convincing evidence". The change is that prior, the defender had to prove it was self defense by "a preponderance of the evidence". The burden was on the defense with a lower threshold and now it's on the state with a higher threshold.
But that's only for the immunity hearing, which is optional. Zimmerman for example, chose not to request an immunity hearing and to go straight to trial. Whether or not there is an immunity hearing, if it goes to trial the state has the burden of proving a crime "beyond a reasonable" doubt, as always. The same as is true in any criminal case.
That standard, the standard for conviction in a self defense case, is much higher than the standard in the hearing. The state could prove their case in the hearing, proceed to trial, and fail to meet the higher burden at trial. That's the one that matters, and it has not changed.
Thank you for the thorough explanation of the legal issue here. I do appreciate the clarification.