Using the documented procedure to move JIRA data from one database to another can fail if you have users where their name only differs in case, e.g 'admin' and 'Admin'

When you follow the steps given in http://confluence.atlassian.com/display/JIRA/Switching+Databases#SwitchingDatabases-MigratingJIRAsDatatoaDifferentTypeofDatabaseServer to switch databases you may get duplicate key exceptions. This will occur when switching from a database that does case sensitive searching by default(such as Postgres) to one that does case insensitive searches (such as MS SQL Server).