I bought this to play over LAN with a friend but cannot due to the GOG installer using a generic key. I don't see a serial key anywhere on the game menu in my account like some other games do. Is there any way to change the CD key after installing?

Thanks. I'll fiddle with it and post how it goes.

Edit: Deleting the key only generates an error about a bad serial. It doesn't prompt you to input another serial key. The serial goes through some kind of conversion for the registry key.

Edit Again: Someone on Github posted a solution to convert keys to the hex format for the registry key entry.

The last post by liskin (Jul. 6th, 2019):
https://github.com/ValveSoftware/Proton/issues/767
Terminal Command, replacing 1234-etc with a real key:
"echo 1234-56789-ABCDE-FGHIJ-KLMNO | perl -ne 's/-//g; tr/IO/10/; for $i (0..2) { $res = 0; for $j (0..7) { $res += index("0123456789ABCDEFGHJKLMNPRSTVWXYZ", substr($_, $i * 8 + $j, 1)) << (5 * $j); }; printf("%010x", $res); }; print("\n");'"

GOG should still provide unique keys and let you input at install or offer a key changer but this is a workaround in the meantime.

Nice find!

It may work only for Steam keys, not BIS original ones, generated before 2010 year. But it is not accurate)))

Update: yes, I checked and found that Arma-1 keys are stored in registry according the same rules as for Arma-2 etc.