Following invitations heading out to developers, Google has now been sending invitations to existing Glass Explorers, offering them an updated unit in exchange for their current product. Owners aren’t required to make the swap, of course, but doing so will bring about some welcomed changes, not the least of which is the ability to use Glass with prescriptions glasses. Those who receive the invitation will have until February 5 to make up their mind, at which point the one-time offer expires and they’ll presumably have to wait until the next upgrade period. The updated Glass units are still Explorer Editions, and are necessary to take advantage of some accessories and features that will be coming out in the upcoming months. If you weren’t completely enthused with your Glass’s color or you want to give a new style a try, you can select a different color option than your original one.