if two foot is too tall,,,,how high is your grow room,
if your plants are sativa's!, then flower the suckers off now, couse they gonna more than double in size before they finnish.if you grow under a less intense light they will grow slower and with more internodes, then as you change from grow to flower, the plants get a boost of light going to HPS,
IF you change them now! the drop in lumens could send the plants into preflower and maturity!then you could fit your HPS bulb to flower off!
good luck!