John bought a used 2012 MacBook Pro for $280. He upgraded to an SSD drive, but the internet recovery tool for OSX put his laptop back to Mountain Lion. Leo says that's how network recovery works, and John later upgraded back up to Mojave. Leo says John did it right: Get rid of the old drive and replacing it with a new one.
Once he's installed Mojave, the network installer should automatically use Mojave moving forward. But Leo says it's a good idea to have the key anyway.