I don't think anybody has to change their appearance in the least, unless, they want a role where the character they are about to portray, is a bodybuilder, and the person has no muscles showing. But in that case, no one would be surprised to hear that, and the person would have to be stupid to expect the role if they are fat.
Carrie Fisher's change for the movie was not very neccesary. She aged a certain way, Princess Leia could have done so as well. But it is no surprise that she wasn't suitable with her curent appearance. It's Hollywood after all.
Even a documentary on Modelling I once watched, showed two trainers who had a to tell an aspiring model that she was still too big for the job when her waistline was 24 inches. 24 INCHES PEOPLE. In my opinion, she didn't look all that healthy, but that's the job she wanted. And she wanted to look like a skeleton.
I have no problem when people are naturally that skinny, it's not their fault. They may not want to be like that but can possibly, not help it. Same thing when people are obese.
For the rest, actors would need to take a stand, and categorically refuse to change their outward appearances when it's not fundamental to their character. But alas, it will probably not happen.