Convert MyISAM to InnoDB in PXC master master
My comments on converting from MyISAM to InnoDB:  http://mysql.rjweb.org/doc.php/myisam2innodb

My comments on Galera (PXC):  http://mysql.rjweb.org/doc.php/galera

ALTER, on Galera/PXC, comes in two flavors;

⚈  Rolling Schema Upgrade = manually do one node (offline) at a time
⚈  Total Order Isolation = Galera synchronizes so that it is done at same time
     (in the replication sequence) on all nodes.


Further discussion (in addition to the manual, and my doc):  http://www.codership.com/wiki/doku.php?id=rolling_schema_upgrade