You don't have to go to 24/0, but it'd probably help they revert a little faster than 18/6.(a few days) If you go with 24/0, I'd switch back to 18/6 as soon as you see new veg growth starting. Hard to describe what to look for, but I think you'll be able to tell when they've started vegging again, 'cause new growth/foliage will be sprouting from the budsites, the buds themselves, rather than the buds just filling out as they would in bloom. Or, you could just keep them vegging on 24/0. It's your choice, but all of them will work.