No change to current gear. If they add new gear for each level between 51 and the new cap I'd be extremely surprised, but it would most likely be quest rewards with stats similar to NQ crafted gear, or HQ if it's aetherial (pink from levequest coffers). In that case, quest reward gear with required level 55 would have the same stats as a NQ piece of i55 Militia (crafted) gear, and the aetherial version would be equal to the HQ i55 Militia gear. More likely though, level 51+ quests and leve coffers would just award current gear (like i60/Darklight/Myth/etc.) to increase power levels closer to the new maximum faster.