Hi Andy,
This very much depends upon who you talk to. The overwhelming response you will get is that diabetic eye damage is non-reversible. HOWEVER, many (myself included) believe this to be untrue. Dr Richard Bernstein claims to have reversed many of his own diabetic complications, and does the same for his patients. He believes that if people can maintain normal blood sugars, they will not only halt many complications, but actually reverse many. He advocates the management of both Type 1 and Type 2 diabetes through a low carbohydrate diet, which drastically reduces the swings in blood sugar levels that cause both hypos and complications. Have a search for him, and also look up Dr Troy Stapleton (an Australian doctor diagnosed with Type 1 in adulthood). He follows Richard Bernstein's methods, and I find his YouTube videos warmer and easier to relate to than Dr Bernstein's extremely clinical and 'matter-of-fact' approach.