The original message was about adding a camera by IP (which we can do) ... i.e. but, as you said, we do not allow you to change the IP of a camera.
Internally, the system needs some kind of id to use to track a camera. When a camera is found automatically by scanning the network, that id is the mac address of the camera, and it can persist across IP changes. If you add a camera manually by IP, the IP address is the only piece of information we have to track the camera internally -- so if it changes we're fundamentally going to treat it as a different camera.
We might eventually allow you to do something like clone a camera and edit its parameters such as IP address, saving much of the repeated work when an IP changes, but the result would still be a "new" camera as far as the system is concerned.
You didn't mention why the automatic network scan does not work for you, but you cannot use it and if you're spending a lot of time redoing things due to IP address changes, please consider configuring your DHCP server to always assign the same IP address to each of your cameras. That will be less painful than having to update things in Camect every time there are changes.
Re: password changes ... You can change the username and password for a camera, even a manually added one, if the camera correctly indicates that the password is incorrect when it's incorrect. The password fields are disabled unless the camera is actually saying that the password is wrong. More camera manufacturers do manage to return the correct error code when a password is wrong although occasionally you'll find cameras that don't.