Every Premium Bond has exactly the same chance of winning as any other. You are not increasing your chances of winning by cashing old ones in and buying new ones. In fact you are diminishing your chance by having to wait a month for the new ones to go into the draw. There is a commonly held misconception that newer ones have more chance of winning because they appear more frequently in the list of winners but this is just because there aremore of them about, older ones having been cashed in etc.
Whatever you choose to do, Good Luck! :o)