Some people leave it turned down for a week or 2 at the beginning of flower, I go one week at 75% then 100 until a week before harvest when I go to 75 again in an effort to preserve trichomes. I don't know that the late dimming helps or not, but I don't think it could hurt. Also, as far as I know the bulb swap generally happens at the same time as the swith to 12-12.