do a full Export/Import of devices to all network nodes.
This is most likely my issue - I was not aware I needed to do any type of import/export.
Where can I learn how to do this properly?
To do an Export/Import? It is quite simple actually, in DB Studio (the programming software) at the top open the Operations drop-down, select Export/Import devices, then in the resulting window select all nodes, select all types of information, and then Export, when that is done, Import. Make sure to do it in that order, because after an Import you must restart the programming software. Make sure you are not logged into other nodes when doing this, the process will fail.
For other maintenance, contact your vendor for training or have them sponsor you to an official Mitel system administration course.
This makes me nervous - will any good data get overridden?
All it is doing is telling the other systems in the network what devices are where, it doesn't over-write anything on the remote nodes, if there is a conflict (which there likely will be for some things, it's normal in some cases) the local system data has precedence. You are basically just updating there "where is it" database, so when you add x1234 in Node 4 until you tell nodes 1, 2, and 3 where it is they won't know and will consider 1234 an unused extension, once you Export/Import the remote lists come into sync and Node 1 now knows "Hey, the user dialed 1234, that's in Node 4 so I need to shoot it across the Intertel Networking link to IP XX.XX.XX.XX". In other words, it is just updating it's remote device database.
This is not an automated process, and should be done whenever extension are added or taken away, and by "extensions" I mean anything with an extension number such as phones, phantoms, hunt groups, applications, mailboxes, etc. otherwise remote systems don't know about the addition/deletion.
This is a safe process.