Which court case held that an old bargaining agreement no longer applies to a newly severed bargaining unit, and an old agreement does not prevent the employer from negotiating with the craft union on behalf of the new bargaining unit

Gyou are in charge of making the work schedule for the next two weeks. typically this is not a difficult task as you work at a routine 8am – 5pm company. however, over the next two weeks you are required to schedule someone to be in the office each saturday. after contemplating this for a few days you make the schedule and assignments. before posting the schedule for everyone you decide that it is a good idea to meet personally with the two people you have scheduled to work the weekend. what do you say to them? what is your desired outcome?