Trace copies the Ability of the current opponent. Trace cannot copy Trace, Multitype, Illusion, Imposter, or Stance Change; however, Trace can copy Wonder Guard. In a Double Battle, Trace will randomly copy either opponent's Ability.
as you see can't trace copy stance change so you will either receive a "but it failed" message or nothing will happen. simple answer but thats how it is
hope I helped:)