Hi. You don't change from T1 to T2 or vice versa but mis-diagnosis is common thru ignorance, lack of tests and the rather silly definition of T1. T1 is lack of insulin production and needs insulin. Although most common in childhood it can and does frequently occur in adulthood. Many medics, for example my diabetes GP, don't know this so label the diagnosis as T2 without any tests. T1 is also rigidly defined only as an auto-immune disease which I believe to be rather silly. It can be caused by viruses as well and has the same effect i.e. destruction of the beta cells. This makes the GAD auto-immunity test rather limited in use. The result of all this is that approx. 15% of T2s are probably Late onset T1 (like me). So, the important thing is there is any doubt is to ask for a c-peptide test which measures insulin output. If low it implies T1 and if high it implies T2. True T2 rarely occurs before middle age unless you are grossly overweight. Being slim can imply T1 at any time due to the body burning fat as it can't burn carbs. Sorry for the lengthy reply correct diagnosis is a problem area within the NHS.