The documentation shows that it is easy to create a new region with redundancy.
However, the alter region command doesn't show any way to modify the redundancy on a region you're already using. Is this possible?
As you correctly stated, there's no option exposed through the alter region command to change the number of redundant copies.
A simple workaround, however, would be to export the region data, destroy the region, re-create it using the new number of redundant copies, and import the data into the newly created region from scratch.