Honestly this should be possible to do with just a firmware update not a new device, especially for the Time Capsules that already have a harddrive.
possibly, if there is sufficient RAM, etc. to support the more complicated OS required. But it's possible there will be a firmware update for the old time capsules/airports as well. The hardware may be updated for other reasons (cost, manufacturability, reliability, power consumption, performance, etc.) - you didn't expect Apple to keep selling the same device forever?