2 employees with less seniority have been put in my department and now I have been forced to a different shift

So go to your union rep and file a grievence if you are union.

Do you work under any employment contract ?

What is your company policy or written rules on job assignment.

Where I work and most places, they can move employees from shift to shift any time they want, regardless of any seniority.

If the only way shift work is decided is by bidding on the job and that is assigned by seniority, then file a complaint with HR