Botanicare Pure Blend Pro Bloom

I am growing in a soilless setup and I picked up the PureBlend Pro Bloom (2.5-2-5, NOT the soil formula) and in the directions, it reads, "Optional: 2-3 weeks before harvest, switch to Pure Blend Pro Bloom Soil (1.5-4-5)" What does having lower N and higher P 2-3 weeks before harvest do for the plant? Also, since the last 2 weeks up until harvest are flushing, if I was going to switch to the soil formula, would I do it, 4-5 weeks before harvest, then flush the last 2 weeks with plain water?