Two weeks ago may be doable from the company side IF you gave them proper notice. From the union side, it may be a bit dicier. The issue is obviously seniority and unless there is a provision in your CBA, (and very few if any have one) in order to gegain your seniority, each crewmember who was junior to you would have to agree to it because your regaining your number is negatively impacting their rights and just doing it could lead to a DFR lawsuit by any or all of those crewmembers.

Best of luck.