switching to 12s

Should I leave girls n dark for 24 hours before switching

That's not necessary
but you can do this if you wish it will not hurt them
good luck

I found that my plants "respond" well to 72 hours of darkness. After 3 days of no light, the plant's "structure" changes a bit as the plant orients itself to start flowering.

Not really sure what the difference is between the two techniques, but giving the plants a 3-day rest gives them a chance to "adjust" before the lighting shift and upcoming bud growth.