I agree with @Foreigner, the auto is gonna do its thing regardless so make sure your photos are getting the bulk of the light and attention. And if she makes it through, bonus weed
I do veg tent autos and you can let it ride while you veg and fill out a canopy. If your veg is a lot quicker than your auto is finishing they will do alright in 12/12 if you flip early, worst thing that happens is you lose out on yield on the auto
From someone who’s never grown autos - can you move the plant up to get more light/DLI after the flip, to help counter the lowering of hours down to 12?
If light is strong enough, one can use a dimmer to set down the power and height of the light in veg for the ideal DLI and turn up the dimmer to compensate the shorter light cycle… as for autos, they can handle 24, but do not need all of that… The DLI for photos and autos seems the same for me…