• ON DELETE CASCADE: When you delete rows in the master table referenced by other tables, all those child tables will also delete their rows that reference the deleted master data.
• ON UPDATE NO ACTION: If you try to update a field in the master table referenced by others, MySQL will prevent you from doing that. NO ACTION and RESTRICT are synonyms in MySQL.

So to answer your updated question, MySQL will prevent you from updating a field in the master table that is referenced by any child tables because you specified NO ACTION. If you are updating any other field in the master table it will take the same amount of time as an update on a table without foreign keys.

WARNING !!!

Before you do anything, mysqldump the entire database or tarball /var/lib/mysql

If you are concerned about case sensitivity, you need to hunt down the usernames in a special way

I tried this little experiment

use test
DROP TABLE rolando;
CREATE TABLE rolando
(id int not null auto_increment,
name varchar(20),primary key (id));
INSERT INTO rolando (name) VALUES
('rolando'),('Rolando'),
('ROLANDO'),('ROLANDO'),
('rolando'),('rolando');

I loaded the sample data

mysql> use test
Database changed
mysql> DROP TABLE rolando;
Query OK, 0 rows affected (0.03 sec)

mysql> CREATE TABLE rolando
    -> (id int not null auto_increment,
    -> name varchar(20),primary key (id));
Query OK, 0 rows affected (0.07 sec)

mysql> INSERT INTO rolando (name) VALUES
    -> ('rolando'),('Rolando'),
    -> ('ROLANDO'),('ROLANDO'),
    -> ('rolando'),('rolando');
Query OK, 6 rows affected (0.08 sec)
Records: 6  Duplicates: 0  Warnings: 0

I ran these queries (please note subtle differences)

mysql> SELECT name,COUNT(1) FROM rolando GROUP BY name;
+---------+----------+
| name    | COUNT(1) |
+---------+----------+
| rolando |        6 |
+---------+----------+
1 row in set (0.00 sec)

mysql> SELECT name,MD5(name),COUNT(1) FROM rolando GROUP BY name,MD5(name);
+---------+----------------------------------+----------+
| name    | MD5(name)                        | COUNT(1) |
+---------+----------------------------------+----------+
| ROLANDO | b3f7ba680fe83ab0b5174737e8d536a2 |        2 |
| Rolando | e0732d78dc135b8fcc33ec79bfa89d1f |        1 |
| rolando | e80d4ab77eb18a4ca350157fd487d7e2 |        3 |
+---------+----------------------------------+----------+
3 rows in set (0.00 sec)

mysql> SELECT name,MD5(name)=MD5(LOWER(name)) from rolando;
+---------+----------------------------+
| name    | MD5(name)=MD5(LOWER(name)) |
+---------+----------------------------+
| rolando |                          1 |
| Rolando |                          0 |
| ROLANDO |                          0 |
| ROLANDO |                          0 |
| rolando |                          1 |
| rolando |                          1 |
+---------+----------------------------+
6 rows in set (0.00 sec)

mysql> SELECT name,COUNT(1) FROM rolando GROUP BY name;
+---------+----------+
| name    | COUNT(1) |
+---------+----------+
| rolando |        6 |
+---------+----------+
1 row in set (0.00 sec)

mysql> SELECT name,(name LIKE lower(name)) like1,
    -> (name = binary LOWER(name)) like2 FROM rolando;
+---------+-------+-------+
| name    | like1 | like2 |
+---------+-------+-------+
| rolando |     1 |     1 |
| Rolando |     1 |     0 |
| ROLANDO |     1 |     0 |
| ROLANDO |     1 |     0 |
| rolando |     1 |     1 |
| rolando |     1 |     1 |
+---------+-------+-------+
6 rows in set (0.00 sec)

mysql>

Give this view of my experiment, a username with all uppercase or mixed case can be equal to an all lowercase UNLESS YOU USE BINARY operator with the comparison

I do not think BINARY can be applied to ON DELETE CASCADE.

However, you could something like this: To delete all usernames that are lowercase only, try running one of the following:

DELETE FROM logins WHERE username = BINARY LOWER(username);

or

DELETE FROM logins WHERE MD5(username) = MD5(LOWER(username));

If you run this now, you might nix all usernames in the other tables. Run this instead

SET FOREIGN_KEY_CHECKS=0;
DELETE FROM logins WHERE username = BINARY LOWER(username);

or

SET FOREIGN_KEY_CHECKS=0;
DELETE FROM logins WHERE MD5(username) = MD5(LOWER(username));

You will have to go to all tables that have a username in it and execute

SET FOREIGN_KEY_CHECKS=0;
DELETE FROM <whatevertable1> WHERE user_name = BINARY LOWER(user_name) AND user_name REGEXP '^.*[A-Z].+$';
DELETE FROM <whatevertable2> WHERE user_name = BINARY LOWER(user_name) AND user_name REGEXP '^.*[A-Z].+$';
DELETE FROM <whatevertable3> WHERE user_name = BINARY LOWER(user_name) AND user_name REGEXP '^.*[A-Z].+$';
.
.
.
DELETE FROM <whatevertableN> WHERE user_name = BINARY LOWER(user_name) AND user_name REGEXP '^.*[A-Z].+$';

Since you have a ton of other tables, use the INFORMATION_SCHEMA to script it for you:

DELETE_SQLFILE=/root/SensitiveDelete.sql
MYSQL_CONN="-uroot -ppassword"
echo "SET FOREIGN_KEY_CHECKS=0;" > ${DELETE_SQLFILE}
mysql ${MYSQL_CONN} -ANe"SELECT CONCAT('DELETE FROM ',table_schema,'.',table_name,' WHERE user_name = BINARY LOWER(user_name) AND user_name REGEXP ''\^.*[A-Z].+$''\) FROM information_schema.columns WHERE column_name='user_name'" >> ${DELETE_SQLFILE}
cat ${DELETE_SQLFILE}

If the file is correct to you, run it in the mysql client like this

mysql ${MYSQL_CONN} -A < ${DELETE_SQLFILE}

Give it a Try !!!