From Sigma:
Since 2013, The SIGMA Corporation has been offering a “Mount Conversion Service” for SIGMA’s Global Vision (Contemporary, Art and Sports) lenses. With this service, the mount and key components inside your lenses can be replaced, adjusted and optimized for another camera mount.
Since the introduction of this service, we have maintained the cost which was set originally to ensure the best possible customer support. This service requires not only a simple mount replacement, but also a detailed assessment of the individual lenses we receive from our customers.
Due to the growing complexity of the conversion service and the increasing cost of required parts, we are having to revise the cost of our Mount Conversion Service.
We appreciate your understanding and continued support for our company and products.