as packetfloss said, you can switch whenever you like. your in control of the morphology of your crop. MH will keep them smaller, bushier with shorter internodes. HPS will cause stretch, growth and flower size. You can grow under a 4000-6000K MH the entire grow and do ok. the reason to use MH during the flip is to keep stretch to a minimum, and harden them up for HPS use in the 4-6 week period of flower growth. im going to assume you have height issues so, MH till 2-3 week flower, HPS 3-7 weeks and finish at least the last 10 days under MH 6000K or higher. you'll have your best most resinous crop yet.