I don't advocating just in general telling actors to change their appearance dramatically if the role doesn't call for it, but then they can't cry foul if they weigh 300 pounds and the character is supposed to be anorexic. If the story is about a football player and you want the job, you need to have an athlete's body.
Yes, actors may have to change their physical appearance dramatically when assigned some new roles. This is part of their acting career. If they don't want to make these changes, they should feel free to turn down a role that is offered them. They are freelancers choosing the work they take.
I don't think that any actor or actress should have to do anything to their appearance that could seriously influence their health and put them at risk, but yes, I think it's okay to change their appearance. They're signing up for a job, so if they get the part of a blond and they have black hair, they should be committed enough to change their hair color.
If a certain person is needed for a role, I think it's the actor's job to change their appearance accordingly, within reason of course. I only think these changes in physical appearance should only be implemented when absolutely necessary, though. There is no need to change certain characteristics that may not fit the actor if it is not absolutely necessary.