I personally believe that once you get to a light level, you stay at that light level permanently until you increase it. Ex: You get to 334. You then equip a year one weapon. Instead of the 170 dropping your 334 down, you stay at 334 since you achieved that light level already. If you get to 335, then you are permanently at 335 but are able to equip your lower gear. This does not only allow you to use lower light leveled gear, but also allows you to equip greens and blues but still stay at your highest light. Infusion would be used still to reach max light, so that function would still exist.