Interesting to see the contract. If, indeed, it says the IBT has exclusive jurisdiction over the Frontier aircraft it will be a tough sell by the company. But, it could say exclusive jurisdiction in Denver, or it could say Jurisdiction (non-exclusive). and other areas of their contract may say something else.
It would also depend on the way they were transfered. If they were terminated and re-hired it could be argued one way, just transfered another way.
This is why you have arbitration.