zpool-upgrade(8) manual page

zpool upgrade Displays pools which do not have all supported features enabled and pools formatted using a legacy ZFS version number. These pools can continue to be used, but some features may not be available. Use zpool to enable all features on all pools (subject to the -o compatibility property).
zpool upgrade -v Displays legacy ZFS versions supported by the this version of ZFS. See zpool-features(7) for a description of feature flags features supported by this version of ZFS.
zpool upgrade [-V version] -a \| poolâ¦ Enables all supported features on the given pool. If the pool has specified compatibility feature sets using the -o compatibility property, only the features present in all requested compatibility sets will be enabled. If this property is set to legacy then no upgrade will take place. Once this is done, the pool will no longer be accessible on systems that do not support feature flags. See zpool-features(7) for details on compatibility with systems that support feature flags, but do not support all features enabled on the pool.
-a
	Enables all supported features (from specified compatibility sets, if any) on all pools.
-V version
	Upgrade to the specified legacy version. If specified, no features will be enabled on the pool. This option can only be used to increase the version number up to the last supported legacy version number.

“	I'm not interested in developing a powerful brain. All I'm after is just a mediocre brain, something like the President of the American Telephone and Telegraph Company.	”
— Alan Turing

Manual Pages — ZPOOL-UPGRADE