Well true, you don't want to stress them out as much as possible when flowering and he's right, that is a possibility, but if you transplant before the lights go out and water with a nice dosage of B1 afterwards, you should be ok, do it quick cuz i know you just started, but honestly, yer plants should be fine in the buckets they're in now. My plants are going all the way with 3 gal buckets. Should be flowering here in a week. But ya, long story short, the grow shop guy is somewhat right, even though it's a small chance, there is a chance of stressing them enough to herm.
check out this thread
https://www.rollitup.org/indoor-growing/60528-transplanting-during-flowering.html