When to start switching nutes?

I have a 6 week old plant who began pre flower stage last week.I plan on letting it veg for roughly another 2 weeks or so.Currently i am using a 20-20-20 plant food during veg.I also have some 20-30-20 which i plan on using for flowering.Now that it is showing sex and in pre flower stage,should i start using my flowering food,or keep using veg food until i put into flowering?

I wait until the plants are making flowers to change over to flowering nutes. I start the early bloom enhancers (open sesame) as soon as I change the light cycle.