I develop an OpenGL-based video engine for a live media playback application, which is very nearly as simple an application of OpenGL as you can expect to find in the real world, and there's no way I could expect to port it to Metal in a week or two singlehandedly. Like others have mentioned, it's a completely different paradigm, not just a matter of changing around some function calls.
That said, I welcome this change with open arms (and secure in the knowledge that legacy code will continue to work for the foreseeable future). OpenGL is a fragmented, brittle, spaghetti-inducing pile of global state. Rewriting in Metal isn't anywhere near as small a project as Apple claims, but I'm perversely looking forward to it -- I'll be very happy to have OpenGL in my rearview mirror.