I am not going to use trigger tags or clean my leaks. This is just for the basic idea of the trigger so you know.
Event-A Unit Starts the Effects of an Ability
Condition-Ability=Grow
Actions:
Custom Script-local unit udg_Grower
Set Grower=(Triggering Unit)
Wait (40) Seconds
Replace (Caster) with (Growed up Unit)
You have a variable there and its Grower, a unit type, then you have my attempt to make it a local variable so it wont change each time Grow is cast, and then Growd up Unit is just the unit that replaces the one that cast the spell. Heres the tut about locals because I am almost positive I got mine wrong
http://www.hiveworkshop.com/forums/showthread.php?t=34393