Sorry if this is in the wrong place, I've searched ACAS and net but can't find a definite answer.
Here's the situation, I've been called as a witness, the case is being heard in another county almost 200 miles from my home town. I was due to be working Mon to Thur off on Fri. My line manager has changed my shift to have me in on Fri, can she do this? 
I don't know when I will be called by the court and if I do get home by Fri I can't guarantee I'd make the shift on time. I need to know if she can legal put me in for a shift none of us have a crystal ball confirming I'll be there for? 
Thanks in advance