Is it possible to change filesystem properties for an existing ZFS pool?

Solution 1:

Sure - zfs set compression=off pool/filesystem (or just zfs set compression=off pool for the pool's root filesystem).

Note that this does not go back and decompress data that's already been written - you'll need to re-write the data to do that (zfs send and zfs recv can help with this).

Solution 2:

Yes, you can do this live. You can even change the compression type...

Remember the distinction between zfs pools and filesystems. Compression properties are set at the zfs filesystem level.

New data to the pool will be uncompressed if you turn it off.

May I ask why you're interested in disabling compression? The recommendation for ZFS is to leave compression ON unless you know your dataset if going to be comprised of non-compressible data (e.g. .tiff files or other media files).