That actually IS the best way to do it.
I use it in many of my maps.
The crash is probably caused because it changed the actual unit to a new one (new handle id)
To prevent that, you have to change the units type id by a transform ability.
Make an ability based of bear form or crow form. Then set the alternative unit type to the original type of your unit and set the standard unit type to the unit with the chaos attack.
Set all stuff like duration, interval, stuff (havent got editor open but you have to set all other stuff to 0.)
Then add that ability to the unit and remove it. DONT issue order to cast it.
By adding the ability, the unit's type id changes to the standard type if he is the alternative type.
Make the same ability but switch the units when that unit drops/sells that item.