There are no down sides, except the little stress from the spontaneous switch in light cycle, but that shouldn't be a problem.
Actually, there's a technique called secondary budding, where 2-3 weeks before harvest you switch back to 18/6. The plant revegs for another week or two. The buds that are already there will continue to grow while the plant continues to stretch and grow. Thus, after 2 weeks of revegging you'll end up with more bud spaces. After that you revert back to 12/12 until she finishes, in some 2-3 weeks. In the end you end up with almost double the yeald.