m0useanswered grow question 2 years ago I would do all my topping/pruning/pinching a week before I flip it to flower. This will give it time to get over the shock of being cut and then enter flower as a new plant.
After that I would avoid pinching off any growth on the top part and just let it grow.
You can veg as long as you want with photoperiods. Just keep the light schedule above 12/12 and it should be fine. when your ready flip to 12/12 and in the next week or two your will notice i start to go into flower if it is mature. Plants need 4-6 weeks to become mature to even enter flower that's why you can grow 12/12 from seed and they don't enter flower instantly because they are not mature yet.
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Best Of Luck!