How do you replace an Airport Extreme N with an Airport Extreme AC without loosing configuration

The AirPort base station's configuration profile can be exported, and then, imported into another base station.

To export a configuration profile:

• Open AirPort Utility, select the AirPort that you want to export the profile from, and then, click "Edit."
• From the AirPort Utility menu, select "File," and then "Export Configuration File..."
• Enter the desired file name in the Save As: textbox, and then, click Save.

To switch profiles:

• Open the AirPort Utility, select the AirPort that you want to import the profile to, and then, click "Edit."
• From the AirPort Utility menu, select "File," and then "Import Configuration File..."
• Using the provided file menu dialog box, locate the file that you just exported; select it, and then, click on "Open."
• Click on "Update" to update the profile.