The stretch period is a huge growth spurt for the plants so they might not be able to pull nutrients in from the soil fast enough and are robbing some of the mobile nutes like N from the older leaves. That high demand will drop after the stretch and go back to feeding levels like they were before the flip. I'd give them a bit more of everything right now leaning toward the bloom side of the equation. Extra light and some CO2 during the stretch really rocks them.
After the stretch I feed at pre-flower levels tapering off while increasing K over P and lowering N to near zero the last 3 weeks or so. I want to see those old fan leaves going soft and yellow as the end nears so I know my plants aren't loaded up with extra nutrient salts when I chop. I don't flush at all but they get bugger all to eat the last couple of weeks other than some Epsom Salts and a little K.
Those lower leaves don't get enough light to be of much use to the plant other than reservoirs for spare nutes the tops can steal as needed so as long as still a bit green and healthy I leave them.