Am I too late to transplant if I’m already in flower?

No my friend, dont do it. The time to re pot is 2 weeks before you switch to flower. this gives a few days for plant to settle and recover and some time to grow out new roots and take advantage of the new space. if you re pot now the plant will get shocked and basically do nothing for up to 4 days, usually only a few but if your rough it can be a few more. So you lose a few days of growth and can even stunt it a bit if that strain doesnt like shock. Then your telling the plant "here- have some space for your roots now" So the plants starts growing roots and focus's less on the buds for a while-thats a double hit to your production now. Most plants will recover from a repot now with only a small drop in yield (never seen a plant not drop yield from a late repot but prob does happen rarely) but some will really hate it and just refuse to do much after and just stay small and look poorly, the buds never really grow much bigger or fill out much more. So not worth the risk is it? The idea to repot is to let the plant grow bigger roots so you get a bigger plant and a bigger yield right? So you need to plan ahead and re pot before the plant out grows the pot its in, but dont repot the plant too soon either as a real small plant in a big pot isnt very effective either. Rule of thumb to be safe- transplant 2 weeks before switching to flower and you be fine pretty much no matter what happens after that.