I had to reinstall dump1090-mutability with piaware, and now I received a new “Site”.
I’d like to “go back” to my previous Site. Both sites are visible in my dashboard.
How could I do that?? I used the same UserID and password!
Already found the answer myself:
piaware-config feeder-id [the old feeder-id]
#Restart piaware
sudo systemctl restart piaware
The above is valid for installations after march 20, 2017.
The old, unused Site will automatically be deleted after 30 days.
piaware-config feeder-id [the old feedre id ([color=#FF0000]MAC address)]
saved feeder-id in file …
#Restart piaware
sudo systemctl restart piaware
#Check status
sudo systemctl status piaware -l
badly formatted feeder id
#Rebooted Pi
sudo reboot
sudo systemctl status piaware -l
failed to start piaware
Yeah, that’s because you’re doing it wrong, you’re trying to use a MAC address where it wants a 128-bit feeder ID. You cannot move MAC-address-based sites in this way and I have said this repeatedly.
That is exactly what I wanted to show 
The method described by rexxbr is applicable for OLD SITES REGISTERED AFTER MARCH 20, 2017. These sites do not use mac address as site id. These are assigned a new 128-bit unique site id by flightaware.
Where site identifier of old Pi is still MAC address, I can still do it with mac address spoof.
I have already described both methods here: