Can't change hostname to our older PBX system via the GUI?

I have an older PBX system, it’s a VM and is called ‘pbx’. We are soon to upgrade our PBX system to FreePBX v14. That VM is called ‘pbx2’. I need to rename the newer pbx2 to pbx for DNS purposes.

When we switch over from the older ‘pbx’ to the newer ‘pbx2’ for testing purposes, I have noticed when I go to change the hostname via the FreePBX GUI, I cant! It doesn’t allow me whatsoever, it does not show an error or it doesn’t crash. It doesn’t allow me to select submit, there’s no change to the hostname. If I change it to something other than ‘pbx’ say ‘pbx3’ for example, that works fine.

I’m confused why this is? I have switched off the older ‘pbx’ VM, so this isn’t on the network, so why can’t I change the newer ‘pbx2’ to ‘pbx’? I have to manually change the hostname in the config files in the command line.