It would be best to switch it using the lights out phase as the buffer rather than lights on, so you'd wait until 7:59am right before lights come on and change the schedule then so that they stay off for another 12 hours before coming back on at 8pm, this will still confuse the plants a little but not nearly as much as the other way around... think about it, in nature there is sometimes very cloudy/stormy days where a plant may never get sunlight so its like an extra long night for them but theres never a night where the plants are getting sunlight, even a full moon isnt enough to keep a plant "awake". Also in regards to your other question about heat, wait until after youve changed the light schedule to determine if you need that, chances are running from 8pm-8am the lights will keep your area warm enough that you dont need to suppliment heat, likewise the temps should stay within +/-10 during lights out since the warmer daytime tempatures will keep it from getting too cold during lights out. most people run their lights at night because of this, unless they have a dedicated hvac system for their room.
My advice for your OP is firstly to take everyone's advice in stride- Yes "blurple" lights are frowned upon by most but that doesnt mean every light that emits that spectrum is junk.. my platinum leds are "blurple" but put out more PAR than same wattage HLG boards, i will be the first to say that no i do not own a HLG board, i do want one and im close to buying a HLG135 for my bonsai moms so i can put the other P150 i have in there currently back into my flower side, but ive seen enough comparison videos and peoples own par readings on them to know that the HLG300 v2 that i considered for my 2x2 flower space has a par of 750ish @18" height and my P300 has 1100ish @ 18" and i can fit 2 P300's or a P300 and 2 P150's in that space whereas i could only fit the 1 HLG300 and nothing else except maybe strips but im not gonna count that since i could add strips to the platinum setup also, and the larger HLGs wont fit a 2x2 space
If youve got some money look into platinum LEDs, otherwise as others have suggested you could look for a cheaper 1000w light that might have an actual draw of 300.. imo thats money wasted but to each their own.. oldschool HPS/MH has never let anyone down either.. if you can manage your temps really well go for that, otherwise id stick to LED for easier temp control.
