You know it would have been easier to just, make separate abilities called Poison level 1, poison Level 2, 3 etc. since upgraded tower as far as game cares is not same unit as base tower. There is no need to trigger it at all. Tower Level 1 gets Poison level 1 ability, Tower Level gets Poison Level 2 ability.
And won't work on buildings, the elevation is in movement and since buildings don't move they don't get elevated. Even units must be set to hover/fly to be allowed to be elevated. Correct me if I am wrong.