employment written warning

Can my employer legally change my availability for working hours without my permission in Michigan?
My employer is trying to change my availability to suit your needs, however, this does not match my current availability, requires different days off, and disrupt my life outside of work, including college and a part-time position. I sent an availability for your request again earlier this year and it was accepted. Furthermore, the availability of new presented no deviation from the job offer and / or the actual program, but simply blocked time would not be available in all circumstances, because of classes and / or my part-time job. I was asked to resubmit my availability and also state a valid reason for not be able to work all hours not included in my availability or face possible termination of employment. I think if I'm done, it would be based solely on this issue as I have never been verbal or written warning and I recently received a merit increase based on job performance.
Unless you have an employment contract "at will". This means they can quit at will or can fire at will. Nothing illegal about it. Do you suck? yes. Maybe they're trying to say something or fail to seek smoking.
Business Management & Leadership Skills : How to Give an Employee a Written Warning


