Under growth turning yellow red week of flower

Been relatively healthy up to this point until I've noticed some older fan leaves in the under growth turning yellow. Not entirely certain what it is as the top leaves have been berging on the greener side . I've been feeding plenty of ppk.

pH 5.8-6.4
Light 480w qb

I have let the soil dry out once or twice but was quickly corrected

Those could use a little defoliation. Very dark green says more than enough nitrogen for now. Was your first feeding after flip a full veg feed?

Think of it two ways. As mentioned above the new growth is very dark, plenty of N available. But the lowers are fading a bit but not looking deficient.

You could think it needs more of something...P or K perhaps, but also mentioned above is how the leaves are on top of each other and may limit the light reaching lower.

Looking at the whole plant you can see plenty of dark green lower down. Unlikely a deficiency.

So the light is not reaching all the branches, some defol is fine or leaving them and just watching how they go is fine too. Losing a few leaves eventually will have a similar effect as a small defol.

Lastly in final pic you can see water droplets on the leaves from sitting on top of each other. Consider trimming some off in cases of direct overlap