When upgrading an online group you should consider the following points:
Regardless of the way which you upgrade your group, it is important to disable any writes to group members until they are ready to rejoin the group.
When a member is stopped, the super_read_only variable is set to on automatically, but this change is not persisted.
When MySQL 5.7.22 or MySQL 8.0.11 tries to join a group running MySQL 5.7.21 or lower it fails to join the group because MySQL 5.7.21 does not send its value of lower_case_table_names.