Can my branch manager switch my pcs orders to go to another duty station ?

They could, as in it is physically possible, but they won't. Once you're on assignment your assignment has passed through about 19 different validation steps. Your branch manager would have to request an exception to cancel your assignment and start the process over for a new assignment. Unless you have a valid reason - an actual need - your branch managers boss is not going to approve that.

Has this happened or is this a "what if" Drill?