When using MySQL 4.1.* and character set UTF-8 then the following error is generated when trying to increase the username field to 64:
[localhost] ERROR 1071: Specified key was too long; max key length is 1024 bytes
The fix was to change the PK to only index the first 100 chars of the namespace field.
ALTER TABLE jivePrivate DROP PRIMARY KEY
ALTER TABLE jivePrivate MODIFY username VARCHAR(64);
ALTER TABLE jivePrivate ADD PRIMARY KEY (username, name, namespace(100));