Most of the work permits that are granted will state that the person shall work for one employer only, and that's the employer that's named on the visa. Nevertheless, work permits can be changed. Some employees actually leave their initial employer and go to work for another employer. There is a process they need to follow to get a new work permit, which means getting a new labour market opinion and then applying for the work permit. So they can indeed change employers, even though the initial work permit will often state that the person can work for that named employer only.
