Worry Seed doesn't affect Aegislash because the ability Stance Change cannot be altered. It can't be affected by Skill Swap, Worry Seed, Entrainment, Simple Beam, etc.
>Stance Change cannot be Traced, copied with Role Play, suppressed with Gastro Acid, or replaced with Simple Beam, Worry Seed, Entrainment, Skill Swap, or Mummy. If a Pokémon other than Aegislash obtains Stance Change with Imposter or Transform, the Pokémon will not change Formes. ~ Bulbapedia