As traced, not all stuff is traceble ....
order by database_name, principal_server_name;
This dmv contains the witness info.
IMO it is fed behind the scenes.
You'll only see the dbmirroring witness service account connect from time to time.
I've also noticed, the dbmirroring endpoint isn't removed after "dismantling" the dbmirroring witness role.
So, I guess, you only need to setup the new mirroring servers to be able to use it.
(a witness can be used by more than one mirroring server pair)
Off course, it's may also be just me, not knowing how to trace dmv feeds.
edited I only found this info regarding witness cleanup ... and that is , you need to perform an alter database on the active database.
Your situation may be worth a MSconnect entry 😎