the battery cannot be changed. the clock inside it would be off and it would no longer work, because it's synced with a server's time (otherwise your token and the server would not come up with the same password).

You have to replace it if it's dying. You'll need to remove it from your account before it's completely dead.

If you already have another token (e.g. from collectors edition xiv) then yes, you can use that. just register it and start using it instead.