I call it a "reverse transform" because I dont know what the real name is.
Here is how it works:
Create an ability based of "Bear Form"
Set all values (especcially requirements) to 0/null.
Put your first unit in the field "Data - Alternative Form Unit"
Put the unit where you want to transform into in the field "Data - Normal Form Unit"
Like this.
Then you go to the triggers.
When you want to transform, do this:
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Actions
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Unit - Add Bear Form to (Triggering unit)
-

Unit - Remove Bear Form from (Triggering unit)
Then your unit is instantly replaced.
You have to make the same ability again with the units switched and use that one to transform back.
Differences from replace:
Replace creates a new unit. With this the unit is still the same.
With this you keep every data that is attached to your unit.
You also keep your spells.