How quickly should courts change the law? Florida tests the limit.

Respect for precedent has been a cornerstone of the American legal system. But what happens if, to “fix mistakes” in the law, courts favor rapid change that the public could not have anticipated?

When the Florida Supreme Court ruled late last month that a unanimous jury is not required for the state to hand down a death sentence, the decision reverberated in the state prisons that house hundreds of felons already sentenced to death.

The legal wrangling over their fate overnight became “chaos,” says Marty McClain, a death-penalty lawyer.

After...