transplanting is really the only way to go.. you're going to seriously shortchange yourself for all the effort this far if you don't.. especially w/ those beautiful roots! those will seriously kick some ass after the flip.
what I haven't seen is a reason why you aren't comfortable w/ doing it, other than your count under the bulb.. I'm guessing no sq ft floor space for bigger pots? if so, maybe you can go up, and not "out." Cut the bottoms out of your current "pots" (the buckets you've got) and buy new ones, fill 'em w/ your medium of choice and stick the plants (still in their original and freshly bottomed buckets) on top. Adjust height by quantity of new soil in bottom bucket.. maybe half?