According to the Storage Spaces FAQ from TechNet you cannot change resiliency once a storage space has been created:
What properties can I change after creating a storage space?
Upon creating a storage space, certain fundamental properties are locked in place. These fundamental properties include the provisioning type (thin or fixed), the resiliency type (simple, mirror, or parity), the number of columns the storage space striping requires, and the striping interleave.
After creating a storage space, you can change properties that do not affect fundamental characteristics, such as the size of the storage space and its name.