Survival Following Termination of Basic Agreement. Unless eliminated or modified following an impasse in bargaining, Article XI shall remain in full force and effect after termination of this Agreement; provided, however, that disputes arising after the termina- tion of this Agreement related to the legality or validity of unilateral changes of terms and conditions of employment following an impasse in bargaining and any other self-help conduct of the Parties, including but not limited to, unilateral changes in nonmandatory subjects of bar- gaining, shall not be subject to Article XI.