It depends on what you mean.
Can you change the actual host name in the masthead? No. You have to generate a new masthead and switch them out on all clients to do that.
BUT, you don’t actually need to change the hostname in the masthead as long as you still control the domain/DNS within the masthead.
You can take the FQDN in the masthead and make it a CNAME entry in your DNS and point it at either a Relay or the Root Server. If you point it at a Relay, this is what is often called “fake root”. You can also point it at the new FQDN of the Root Server itself, so in that case the root server will be reachable by both the old FQDN and the new FQDN.
The reason this matters is new clients will only be able to reach what is in the masthead at first unless you provide them with a clientsettings.cfg or a relays.dat file or both that configure something else for it to talk to.