You can switch to bloom when you start to see pistils stacking up.
If you get a little bit into flowering and start to see yellowing, add back a little veg nute.
There's nothing that says you need to be exclusive with one nute or the other during flowering.
If you've got sativas, you might want to add a little bit of veg nutes to the bloom nutes throughout the first few weeks of flowering, they eat a little more calcium and nitrogen than indica dom strains