For each chunk of on-disk data, the fs stores which devices it is stored on and the used parity configuration. You can take one such chunk of data, and clone it into a new chunk with identical contents but a different parity configuration in the free space of the devices that are part of the file system. (Just like you'd allocate new chunks for storing new files in the same parity configuration). Once that copy is created, all references to the old chunk are changed to point to the new chunk and so the old chunk is now free space. Repeat this process for all chunks in the file system, and the whole file system is converted to use a different parity configuration.