Before today I only read about people who wanted a C-mount option for the GoPro models. I wondered a couple of times how long it would take for someone to come up with a marketable solution for this. I guess not that long.
The Canadian company BACK-BONE has taken it upon itself to produce and sell a modification kit for the GoPro Hero 3 Black, turning it into an interchangable C-mount lens camera with a 1/2.5" sensor (which results in a 5.7x crop factor (to 35mm equiv.)). The mod replaces the two-part front-cover of the GoPro and has a removable IR filter. There isn't much more to it, it is really simple and effective.
They are planning more features like an adjustable flange distance, lanyard attachment and CMOS centered tripod mount.
The upgrade isn't available just yet.