ok lets break this down into sections.
swapped to 12/12 from what? 20/4?
2 days ago you saw brown tips and now brown spots?... where? on the edges of the leaves? middle? see if this helps:
View attachment 1892180 if you look closely, youll notice theres really no necrosis on the tips/edges of the leaves. usually thats a sign of over ferting (the leaf hangs downward, the heavy elements make their way to the tips, gather in number, and start to burn it.) the other possibility to brown spots all over leaves is bugs. thats something you need to check for.
Having trouble telling if you have white hairs or new growth: Day 4 into 12/12 thats normal. day 7 into 12/12 thats normal. hell, could be 2 weeks before you see hairs. but when they are there, you'll know, early flowering, the hairs are hard to see, perhaps just a few milimeters, but after a week or 2, they will stick out around and inch. either way, dont look for hairs, look for balls. even if you see hairs, keep looking for balls, on every internode, up and down every plant.
what you dont wanna see is this:
View attachment 1892181 if theres hairs, your good. balls its male. a mix of both, you got lady gaga and you need to snip the balls off (or keep them for feminized seeds!)
Pics hard to take cuz of work: dude ive come to realize one thing. turning the lights on for 1 minute to take pics and get answers that are closer to the truth is better than people, like me, guessing. hell, break off a fan leaf at the top, middle and bottom atleast, and take pics of those.
So yea: where to go from here: take pics. check pH. if you can check ppm, do so. if not, cut back a bit on nutrients (til we see pics). check for bugs. check for ballsacks.
good luck