To resolve this issue,find and rename or delete the proper license.properties file from your system.
In Windows 2000 and Windows XP the license.properties file is located in:
..\Documents and Settings\All Users\Application Data\Adobe\Flex
Note: If the file is not under "All Users," it may be under a specific user account.
In Windows Vista, the license.properties file is located in:
In Mac OS/X, the license.properties file is located in:
Once you have renamed or deleted this file, you will be able to enter your new serial number the next time you start Flex Builder.