If Snapvine targets a unit (Kai'Sa) but the target is removed with Retreat before resolution, does Snapvine retarget another unit?

Ruling: No, Snapvine does not retarget. Once a trigger has finalized and the target is chosen, it cannot be changed regardless of what happens to either unit.

Nuances:

• This applies even if other valid targets are available on the battlefield
• The targeting decision is locked in when the trigger finalizes, before resolution