So my boy is now 6 months and wondering if its ...

So my boy is now 6 months and wondering if its necessary to switch to a follow on formula? Or should I continue with what he has always been on? It says on the back of the tin you can keep them on number 1 formula after 6 months so why is there even follow on formula? What did other mums do and how did bub go transitioning on follow on? He has Nan Pro Gold 1 at the moment, should I switch to Nan Pro gold 2?