Can an employee be fired for an availability change?

I work at a Marshalls, and I have told my manager in the past that I can only work on Sundays. Recently, I have been scheduled to work Saturdays. I would like to address this and reaffirm that I can only work sundays but I'm worried that my boss will think I'm not available enough and fire me. How likely is this to happen? Has this actually happened to anyone else?