Not so easy. Working on it.
Edit: Hm, again took me only 5 mins
ChangingOwner
Events
Unit - A unit Changes owner
Conditions
Actions
Game - Display to (All players) the text: ((String((Unit-type of (Triggering unit)))) + ( changed the owner from + ((Name of (Previous owner)) + ( to + (Name of (Owner of (Ownership-changed unit)))))))
But I think it's a bit difficult to understand. If you don't understand it, I'll give a more detailed description. Small hint: concatenate strings