I'm pretty sure that I first read the story in a sci-fi collection.
I think the story takes place on a space station. There is a businessman who wields a great deal of power, who has managed to track down a man that can manipulate himself at the molecular level to change his appearance, age, health, etc.
The businessman is old and wants the secret and goes about trying to convince the other man he needs the secret, so he can live and do a bunch of good for humanity using the technology his company is currently developing.
The immortal man decides to test the businessman by teaching him part of the secret to see how he uses it. The businessman fails after giving himself an overly vigorous, powerful, and handsome form, but neglecting all the little chemical balances within the body.
The businessman dies, and the immortal man changes himself to take the businessman's place to actually do the good the businessman had claimed in his attempt at immortality.