The bird died this week due to liver failure. Amazons have been reported to live for forty or fifty years, and there are some stories of them living even longer. She is devastated by the loss. When I first met the parrot I warned her that the bird was much too heavy and let her know that seeds were a treat, not a sole source of nutrition, for this type of bird. This was only a few months ago, so I doubt dietary changes would have saved the bird's life at that point.
Liver damage is something that progresses over the years and is usually permanent.
Certain parrots are more prone to obesity. These include many of the Amazon parrots and African Greys. The primary source of nutrition for these birds should be in non-seed forms. Pellets There are many excellent pellets on the market. If your bird rejects one, try another.