If they're 2 weeks into flowering already then u just gotta make sure any time changes don't cause dark periods of less than the usual 12 hours - During flowering, giving less light period is far better than giving less dark period or else they can confuse & stress.
Ur better to change times after tomorrow's usual 12pm switch-off & then leave them in dark for 36 hours until 12pm the following day, which is then start of your new 12pm to 12am schedule.
(Tho to be fair, not doing this doesn't mean u've instantly fucked ur plants either - they could quite easily not care about 1 days longer light/lesser dark & they just carry on flowering as normal - just depends how sensitive ur babies wanna be really; but i always prefer to not be too rough & ready with them if i can avoid it!)
Incidentally, during veg stage, i find this info can be reversed - a lesser dark period won't worry them so bad & a temporarily extended light period will generally give them a little extra boost of growing time.
