Wait until there are small "budlets" before changing to bloom nutrients, cannabis still needs adequate nitrogen in early flower.
Besides, a few pre flowers are not the flowering phase yet.
Switching to bloom nutrients too early, just when there are a few pre flowers (primordia) is one of the most common mistakes I see on Grow Diaries and usually leads to people harvesting sickly looking yellow plants when mature, due to stopping nitrogen too early.
Yes, primordia at day 18 is common with autos.
Stick with superior photoperiod plants would be my advice, autos seem ultimately disappointing in my experience.