One frustrating lock-in for interchangeable-lens camera systems is having to give up the expensive stable of lenses you've amassed when you switch to a different camera manufacturer -- other than Micro Four Thirds, supported by Olympus and Panasonic, each company has its own proprietary mount. There are mount conversion adapters, but you frequently take a hit on capabilities or performance when you use them.
Sigma, on a roll with new and excellent lenses that fall under its "Global Vision" marketing strategy (the ones labeled Contemporary, Art, or Sports), adds another compelling reason to buy with its Mount Conversion Service. Sigma's MCS lets you -- for a fee ranging from $80 to $250 -- convert any of these newer lenses to another mount, as long as that mount exists for the lens. So, for example, you could convert the full-frame 35mm F1.4 DG Art from/to a Canon EF, Nikon FX, Sony A, Pentax K or Sigma mount, but not to a Sony E, because that mount isn't offered for this lens.
As technology advances in camera bodies, I think people's eyes stray to greener camera pastures elsewhere a lot more often than they used to. This option certainly makes the thought of shelling out $3,600 for the 120-300mm F2.8 DG OS HSM a little less intimidating, but there's still your set of manufacturer-specific lenses holding you back.
Sigma also took the opportunity to announce a change in its warranty program, extending it to four years for all of the company's products bought on or after July 1, 2013.