Google promised early Glass wearers that they could eventually upgrade to next-generation Explorer hardware, and that moment is finally at hand. The company is now inviting owners to swap devices through a free mail exchange before February 5th. While the upgrade isn't mandatory, Google warns that it's only developing with the newer Explorer Edition in mind -- there's no guarantee that new accessories or software will work on old models. The invitation is more of a push than a gentle nudge, then, but we doubt that many Glass users will complain about a free update. Check out an excerpt from the invitation after the break.