I think the following might give complete information (I removed some
columns not involved in the problem)

Server version: 5.1.49-3 (Debian)

SET collation_connection = utf8_unicode_ci;
Query OK, 0 rows affected (0.00 sec)

show variables like '%colla%';
+----------------------+-----------------+
| Variable_name | Value |
+----------------------+-----------------+
| collation_connection | utf8_unicode_ci |
| collation_database | utf8_unicode_ci |
| collation_server | utf8_unicode_ci |
+----------------------+-----------------+
3 rows in set (0.00 sec)

show variables like '%char%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | utf8 |
| character_set_filesystem | binary |
| character_set_results | utf8 |
| character_set_server | utf8 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)

select USER_ID from TBL_USER where USER_ID = 'GIUSEPPE';
+----------+
| USER_ID |
+----------+
| GIUSEPPE |
+----------+
1 row in set (0.00 sec)

select USER_ID from TBL_USER where USER_ID = 'giuseppe';
+----------+
| USER_ID |
+----------+
| GIUSEPPE |
+----------+
1 row in set (0.00 sec)

update TBL_USER set USER_ID = LOWER(USER_ID) where USER_ID = 'GIUSEPPE';
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign
key constraint fails (`myschema`.`TBL_COMMENT`, CONSTRAINT
`FK4F6E52581590B46E` FOREIGN KEY (`USER_ID`) REFERENCES `TBL_USER`
(`USER_ID`))

| TBL_USER | CREATE TABLE `TBL_USER` (
`USER_ID` varchar(50) COLLATE utf8_unicode_ci NOT NULL,

<< cut >>
PRIMARY KEY (`USER_ID`),
<< cut >>
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |

| TBL_COMMENT | CREATE TABLE `TBL_COMMENT` (
<< cut >>
`ID` bigint(20) NOT NULL AUTO_INCREMENT,
<< cut >>
`USER_ID` varchar(50) COLLATE utf8_unicode_ci DEFAULT NULL,
<< cut >>
PRIMARY KEY (`ID`),
<< cut >>
KEY `FK4F6E52581590B46E` (`USER_ID`),
<< cut >>
CONSTRAINT `FK4F6E52581590B46E` FOREIGN KEY (`USER_ID`) REFERENCES
`TBL_USER` (`USER_ID`),
<< cut >>
) ENGINE=InnoDB AUTO_INCREMENT=7876 DEFAULT CHARSET=utf8
COLLATE=utf8_unicode_ci |