Does official AT&T unlock also upgrade the operating system?
Yes, a backup and restore will force iTunes to update the device with the latest version of iOS. In order to get around it, follow a slightly different procedure described at How to restore iOS without updating (edited instructions here):


  
  Backup the iPhone in iTunes
  Disconnect the iPhone from the computer
  On the iPhone, go to Settings > General > Reset and select "Erase All Contents and Settings"
  Wait for the phone to display the swipe to setup screen
  With iTunes already running, connect it to the computer 
  iTunes will display the unlock message
  Click on Done
  iTunes will present the option to restore from a backup
  Restore from the recent backup you made
  iTunes will also sync all your apps after the data and settings restore is completed (the entire process may take a while depending on the amount of space used on the iPhone)
  


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