Unfree operation is not possible by design choice.
When a requirement reaches a level where it is reviewed and ready to be implemented it is important to not allow changes any more.
This would be bad for the developer as he will never have a solid base for development.
If we would allow unfreezing per default requirement engineers will always unfreeze and freeze again -> no big win.
IMPORTANT NOTICE - 20111029 - New Feature
Give a look to mantis - TICKET 4786: Add right to allow UNFREEZE a requirement