There is no problem.
The difference between Bribe's suggestion and my explaination, is that with Bribe's, you dont make the unit play an animation and probably dont interrupt orders.
In normal WC3 abilities, you have to cast the ability in order to morph.
Neither is better than the other because they are used in different situations.
(A valid comparison could be done on Passive Transformation vs Chaos.)
The Passive Transformation (which could also be used on normal units) is usefull for abilities that change something that cannot (simply) be done using triggers.
For example, if you want your unit to gain attack range, you simply create two unit types and add the attack range to the second.
Then you can switch between them when you activate/deactivate the ability.
Using this method over normal morph means that you dont interrupt orders and the transformation is instant.
You also dont have to have the morph ability available in the UI.
This is activated using triggers (you can use a spell effect event of a dummy ability to make it respond to a certain ability casting).
This is also permanent until you revert it again (using triggers).
Normal morph (which could also be used for heroes) can be used for a unit that just has to morph to something else.
You can use this in most situations and has the regular Object Editor to modify the ability's duration, etc.
This one is activated simply by casting the ability in the UI (or ordering via triggers).
Look at Illidan's Metamorphosis for example.
It is sligtly different than Bear Form or Crow Form but has the same hardcoded set iirc.
However, just try making a custom ability based on Bear Form and set "Hero Ability" to true.