My employer ,in the middle of the work week allowed me to switch shifts with someone who had more hours than me on my shift.

Shift scheduling is usually at the discretion of the manager. As such, changing shifts around happens quite often in many jobs. Also, you voluntarily requested to switch shifts with someone else earlier in the week which means that you requested the initial change to your shift yourself. Without more information, it is difficult to see how the supervisor has done anything wrong based on the changes made above.