Would you have a go? Ifixit have quit a few step by step guides on various models of airport extremes.
I had one of the older 4th gen 2tb models that i no longer used and just last week i took the disk out to "securely destroy the data" . It took me about 2 minutes to open an unplug the disk, a replacement would have been that easy.
This guy in Geneva does a lot of work for people .. I've no personal experience but I've pointed people to him before and they have been happy..