If you are sure you use the same cert, change the namespace to 1.5.2, and remove the publisherID tag from Application descriptor.
Then package the AIR file and see the upgrade works.
If the upgrade works, then you probably don't set the publisherID correctly.
If the upgrade doesn't work, then you have a different cert.
This did not work. The cert is the same as the original. There is definately something different about moving between a OS X 10.5.* machine running Flex Builder 3 to a OS X 10.6.* machine running Flash Builder 4. This is not good.
Packaging is done by ADT tool. This process should not be impacted when you move platform and development enviroment.
Is it possible for you to send the two versions of the AIR files to me? I can take a look at it.