Can a Slann leave one unit of Temple Guard to join another unit of Temple Guard, assuming that you have two units of Temple Guard? Can a Slann walk (float) from one unit of TG to another unit of TG? Rules state he can never leave the unit of TG but does not say anything about if multiple units are on the board
Pg 47 Lizardmen Army book: Each Slann Mage Priest must join a unit of Temple Guard when both are present in the same army...The Slann cannot leave this unit. Basically the slann cannot even make the switch to another TG unit if the current one is alive, since he cannot leave the unit. Once that unit dies the slann must try to accompany the other TG unit according to the rules.