My father worked in the US for 2.5 years on h1b. Had both h1b and b2 in his passport (b2 perceeded the h1b and is valid until 2018). Before he left the US he had filed for COS from h1 to b2 and stayed in the US for a couple of months. The request was approved (albeit after he had already left the US). My question is whether he can still use the b2 visa he originally had in his passport, should he want to visit the US in the future (i.e. until 2018), or does he need to get a new b2?