I was writing a tutorial about shape shifting abilities and as a mechanic in Warcraft a while back, never finished or published it, but here is a quote from said tutorial, and it will probably be the cause of your unit dying:
"In order for transformation abilities to work the "Alternate Form Unit" must have the ability. If the "Normal Form Unit" has the ability, but the other unit form does not, the unit will die upon casting. Alternate Form Unit does not need the "Normal Form Unit" to have the ability - it just works as a one-way-shape-shift only then."