Switch from 400 to 600

Can anyone advise me can I start flowering with a 400w hps and then switch to 600 in a week or so or will this affect bud sites???

that'll be fine. wont hurt anything

Drolove thank you can you also tell me can I now put potential mothers under t8 or t5 maybe or will they stretch???

ya thats fine and as long as you keep your t5/t8s 5 inches or less from the plants they wont stretch. with floros like that the plant can almost touch the plant without burning them

u can chose to penetrate more deeply with the 600,
or you have lift the 600 up, and cover more space with about the same penetration as a 400... your choice, see what works for yourself.