help with changing light schedule/flowering

so i have a plant that is about 21" tall now. sturdy, green and healthy and i think ready to push into flowering stage. I have a few questions about this. First, I just added "veg" nutes(high nitrogen) about 3 days ago. Should i do anything to flush the soil before switching my light schedule from 18/6 to 12/12? Secondly, once i do change my light schedule, should I add my "flowering" nutes (foxfarm products) right away or wait a week or two into the flowering stage. thanks for the help guys

i wouldnt flush them. just go ahead and switch to 12/12 and when they are ready to be fed again then go with flower nutes.
by now at 21" tall you should have preflowers and know what the sex is.
here is a link to see what preflowers are.
https://www.rollitup.org/view.php?pg=faq&cmd=category&id=53