Congestive heart failure, when caused by an obesity thickened heart muscle, as far as the new info I've seen and I can figure, can not be reversed. The extent of damage probably depends on how heavy a person has been and for how long. I quit.1) Know that your body is capable of safely handling weight. Obviously, no one can know for sure, if you've been severely bulimic in the past, for instance, and have latent heart damage, you would obviously endanger yourself by extreme gaining. Karen Carpenter died because she had been severely anorexic and the little weight she gained while recovering from anorexia caused her to die of heart disease, according to media accounts.
2) You obviously can't ask a doctor to examine you to determine if you can safely gain weight, so you might study up on the indicators such as BP, blood sugar, cholesterol, and others so you can at least make an informed decision for yourself.
3) The medical community will almost certainly never recognize that some people really want to be very heavy and will never seek to help us gain safely. We are pretty much an unknown science and are taking our health and lives in our own hands. Doctors and the medical establishment seem to have an acute shortage of common sense, so I think we can use common sense to our advantage, listen to our own bodies, and confound those short-sighted med people by being far healthier than they'll ever believe.
Wear sunscreen, while the rest of my meandering advice...
This stuff is only my opinion. Consult medical professionals.