Its a different model if i recall correctly. You would need to use some kind of metamorphosis spell or chaos.
Otherwise you might just as well replace the unit and create a new one via triggers but you need 2 different unit types with the proper models.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.