Feeding to flowering.

i have some questions getting into flowering. when do i change the feeding? should i keep feeding it what i have been from veg for the first week then change over to my flower feeding? ( my veg feeding is 11-5-5. my flower feeding plans is 3-20-20) do i need to flush out the soil before i start feeding it? any advice appreciated.

just water with regular water for a few days during the switchover then use half doses of the new nutes for the next couple of feedings, ease them into it.