T2 to T1 doesn't happen. Some T2s do require insulin, but they are still T2s. The conditions have similar symptoms, but different causes and one doesn't become the other. You don't define them by their treatment. In any case, it is NOT inevitable that T2 is always progressive until insulin becomes necessary. If you follow the conventional treatments,diets and levels of control, it is very likely. If however, youtake command and CONTOL T2 diabetes( that means blood glucose in the non-diabetic range) you can most likely avoid all that.
Read through this website and you'll learn more. Look particularly at "success stories" and "stickies"
