If you are trying to figure out if its a girl or not the plant will have to be in 12/12. Once it has been in 12/12 for a few days (or more), you will begin to see the pistils (girl), or balls (boy).

Maybe I am misunderstanding what you are trying to do. I am a little confused here as to why you want to switch it back into veg if you just changed it to flower.:wtf:

Are you asking if you can switch it back to veg after flowering and re-flower again?. If so, yes you can, but it may not yield as much as the first so I have heard. If you are trying to switch it back to veg after you have sexed them the answer is no you shouldnt because it may hermie from stress or worse if I am not mistaken.