I get the feeling that the big fan leaves are more for stem growth, and the bud leaves for bud growth. With some of the strains I grow, it seems like the lower fan leaves go yellow pretty quick, but it doesn't seem to effect the bud growth that I notice. It could be that they're not getting as much light as they did before the plant got taller with bloom, maybe it's nutes lol, but it does seem different from strain to strain for me.