I don't see any reason why this won't be possible. Though, I would likely used different sets of cinder-volume services for that, as while Ceph does support active-active setup, I'm not sure your NAS driver does - it's worth checking your specific driver with this matrix:

Regarding changing volume types, I assume you mean basically volume retypes. It actually depends on the format of images that are stored on the backend, as cinder does not execute type conversion during retype. So if your NAS storage stores in RAW as Ceph does, this should work, I assume.