The official rules for Zombivors are now available here.
Death and zombie infection can’t turn a real hero into a monster .
Your favorite Survivors can make it back to the game as Zombie heroes
called Zombivors! When your Survivor is eliminated by Zombies, you
can turn him into a Zombivor: flip his Identity Card and savor a
bloody revenge . The Zombivors’ status is indicated with on the
Survivors’ Identity Card . All Survivor rules apply to them . The
killer feature about Zombivors is their incredible endurance: a
Zombivor is only eliminated by five Wounded Cards in his Inventory
(instead of two for a standard Survivor)
Zombivors are great tanks . There’s a serious drawback, though
(besides the foul breath): at Yellow Level, Zombivors don’t gain the
+1 Action Skill of standard Survivors
It goes on to explain that you can either play with them starting as zombies, or play with them starting as normal but coming back as zombies if they are defeated (which obviously is a big advantage).