Many growers make the decision based on limitations of the grow space keeping in mind that many plants will double in size the first 4(ish) weeks of flowering. Others do it simply based on the desired harvest date and considering that it takes 8 to 12 weeks for flower to ripen after the start of flowering (when pistil appear at the tips of the plant.)
High!! If the large plants are ready and you want to flip them ahead of the smaller ones AND have another location to switch up a light schedule for them go for it!! Space is key to choosing flip times for a bunch of us although you clearly have a ton of space in your tent available. Or you can just wait for the little ones to get to a point your con’tent’ with and then drop to a 12/12.