Change oracle port from 1523 to 1521 when something else is on 1521
Before even posting I found the answer to my question.

As it turns out there should only be one of those processes running.

The first thing I did was try to find out more about this process on port 1521. I took the PID and searched for it.

ps -ef | grep 119935
oracle   119935      1  0 May11 ?        00:00:04 {Old install path}/bin/tnslsnr LISTENER -inherit


I noticed that the path shown here is to a directory that doesn't exist. I had previously installed oracle to this location and then uninstalled and reinstalled (I had to do this twice...). I had even "rm -rf"ed the whole directory. So this process must be from the old install, still running in memory. There's probably a bunch of stuff running that shouldn't be from my hacking so I rebooted the machine.

Now processes are gone:

$netstat -tulpn | grep tnslsnr
nothing here


With port 1521 free I can change the listeners port