Old growth yellowing

Just had a quick question.
I have two plants that are in the first week of flowering. I switched to 12/12 exactly 5 days ago. Before switching, I noticed that leaves on the lower branches were yellowing and dropping off starting with fan leaves. I lollipopped them a few days before I switched because they didnt receive much light anyway and figured that could be the issue. The top canopy still seems healthy I just want to make sure it won’t progress.

Is it normal for them to yellow if they receive little light? Should I look out for something else?
Before switch I used General Organics BioThrive Grow (1/4th dose) about once a week. Since switching they’ve been given 1 watering with BioThrive Bloom (again 1/4 dose).
Soil is FFSF.
Light is WILLS 600w full spec LED with both Veg and Bloom switches on. Unrelated question, should I only use the Bloom switch on the light?

Thanks!