my flowering mango strain has done the same when ive changed from envirolights to a 600w hps light the older buds that were already present have strethed out and you can see stem inbetween the individual buds
my plants in about 7th week too mate so wouldnt worry too much
the growth was stunted a bit due to the pot being too small so even though its probably really in its 8th maybe 9th week its now still makin buds that are really compact il have to give it one week of flushin then next week without any water then harvest should do nicly to fatten up the buds a bit
so keep an eye on em see if new buds look better more compact if so maybe give it an xtra week to fatten up proper
just make sure you keep an eye on the buds and especially the trichomes you dont wont the thc to get past its best by date if ya get me
goodluck
also do you get heat issues with your 600w hps???????????
if not then why? cos i do, my plants are turnin crispy on top (oops)