(Updated 2017/07/16 - now applies to all sites)
PiAware feeders are now being identified by a unique feeder ID stored by PiAware, rather than by hardware MAC addresses.
A feeder ID will be assigned when the feeder first connects, and will be stored by PiAware on the sdcard. Subsequent connections will use that feeder ID to identify the site. The main consequence of this is that if you re-image the sdcard or otherwise remove the feeder ID, then subsequently the feeder will look like a brand new feeder and will create a new site.
If you want to retain the same feeder-to-site association when re-imaging, so that the existing site continues to be used, then you can explicitly configure the feeder ID to use. This is a somewhat manual process at the moment:
Find the feeder ID that you want to use. You can find this either as the “Site identifier” on the site page at flightaware.com/adsb/stats/user/yourusername, or from the PiAware logs in /var/log/piaware.log on your existing install. The identifier looks like a series of dash-separated hex digits: 12345678-1234-1234-1234-123456789abc.
Configure the feeder ID on the new system: piaware-config feeder-id 12345678-1234-1234-1234-123456789abc
Restart piaware: sudo systemctl restart piaware
This also works when replacing hardware (if it is just a Pi replacement, you can just move the existing sdcard over and the feeder ID will be retained)
This now applied to all sites, including sites created before feeder IDs were introduced. Those sites have had a feeder ID assigned retrospectively.
If you are running an older version of PiAware then the description above is not entirely correct (as older versions can’t store a feeder ID, there is a fixed translation of MAC address to feeder ID that happens on the server side now) but the site still has a feeder ID, and you can migrate to new hardware and keep the site so long as you also upgrade PiAware at the same time and configure the feeder ID.