Is there anything worse than some yellowing of the lower leaves? How long have you had them going? From seed or clone? If not, then don't worry about transplanting until or unless they're becoming root-bound.
If it were me, I'd start feeding them weakly, maybe something like fish emulsion, at 25%-50% the recommended dosage for vegetables and herbs, that's got a decent amount of nitrogen in it (you don't need a fert with 20%-30% nitrogen, 5% is fine, and even that can burn), along with molasses and a topdressing of something that has mycorrhizal fungi and soil microbes in it (I really like Dr. Earth as it doesn't smell bad if made into a tea, but am currently using Happy Frog as its cost is slightly less, always dosing every two weeks or so with Super Plant Tonic for a wet live culture of microbes).
See, if you've been giving them nothing but water, then there's little reason for them to suddenly become burned. Nitrogen is a mobile nutrient, so what the plant will do is move it from older (usually lower) leaves into th newer leaves so it can continue its objective, which is to grow.
