My employer is mostly non-union. If I am an existing non-union employee well past satisfying my probation period and my employer assigns me to the same job elsewhere in the company but that assignment requires I join the union, can my employer make me sat

I drive a school bus for a private employer who contracts with public school districts. My employer has one contract with a district in which my employer had to negotiate a CBA with an existing union. I have been non-union until now, but will now be regularly assigned to a driver position that is part of the bargaining unit covered by the CBA. I completed my six month probation awhile ago. Can my employer require I fill another 6 month probation now that I have been reassigned to do the same job. The CBA refers to a 6 month probationary period and that probationary employees aren't part of the bargaining unit. I think it is for new hires only, but the CBA doesn't specifically say that it does and my supervisor says no I can't join the union until I complete six months in the assignment. Doesn't that defeat the whole purpose of probation and of a CBA? Couldn't my employer just fire me, or reassign me back to a non-union position within the 6 months? Seems like an unfair labor practice to me,