This is a pretty basic trigger. You first have to set the unit who you want to change to variable "OwnerChanger" in some other trigger (maybe map initialization). I used the spell Holy Light, but you can ofcourse use whatever you like.
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ChangeOwner Events Unit - A unit Begins casting an ability Conditions (Ability being cast) Equal to Holy Light (Casting unit) Equal to OwnerChanger Actions Wait 1.00 seconds Unit - Change ownership of (Target unit of ability being cast) to Player 2 (Blue) and Change color