No problem. Oh, and If you are replacing the unit while the transformation, then you could use something like.
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Unit - Replace (Triggering unit) with a (Unit-type of (Triggering unit)) using The old unit's relative life and mana
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Hero - Set (Last replaced unit) Hero-level to (Hero level of (Triggering unit)), Hide level-up graphics
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Unit - Set life of (Last created unit) to (Life of (Triggering unit))
And, if you're creating a new unit
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Unit - Create 1 (Unit-type of (Triggering unit)) for (Picked player) at (Position of (Triggering unit)) facing (Facing of (Triggering unit)) degrees
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Hero - Set (Last created unit) Hero-level to (Hero level of (Triggering unit)), Hide level-up graphics
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Unit - Set life of (Last created unit) to (Life of (Triggering unit))
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Unit - Remove (Triggering unit) from the game
All in actions of your triggered spell, and then the only problem would be to make the unit learn every spell it had before, or you will be making a cheap bug.
But i don't know that part, really sorry.
I suggest you, to use the tutorial that WaterKnight told you before, that way is really better.
Also, the problem of the stone flying is because you need to remove that animation after transformation.
So the trigger would be something like this one. Still gotta find the right animation, i could be wrong.
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Animation - Remove the (Gargoyle, Stone Form Gargoyle) animation tag to (Last created unit)