You can upgrade a storage system in an HA pair to a storage system in an HA pair in another family by replacing the controllers and connecting the disk shelves
To upgrade the original nodes, you must follow certain guidelines and be aware of restrictions that affect the procedure
The question was not how to reuse existing shelves, but how to convert FAS2040 into disk shelf. To my best knowledge it has never been supported for FAS2000, but I cannot find reference (FAQ does not mention it).