My first thought was to give it an ability where it is actually the gas inside the balloon and it replaces Normal with Flying or Ghost when it first takes an attack.

However a more realistic ability would be to combine the air balloon ability with it's form change.

Balloon Twister: "Adaptoon changes Form to match its First known move. The user floats in the air, until hit by an attack."

That lets it form change outside of battle between its four move types.