just documenting here for now.

upgrading db might error out like this :

ERROR 1267 (HY000) at line 779: Illegal mix of collations (utf8_unicode_ci,IMPLICIT) and (utf8_general_ci,IMPLICIT) for operation '='

that happens even if global, connection and database collations are set to 'general' one - somehow, somewhere 'unicode' still appears.

a similar case was reported as http://bugs.mysql.com/bug.php?id=24690 - but that seemed to cover exactly the opposite...

so far the only solution i've found is to modify collation for the database (which might not be always possible/desirable) :
alter database zabbix collate utf8_unicode_ci;