Some repos, such a the one for the Unifi Controller, use different ‘field’ values to tie a release and require manual updates. For someone like me who has a standalone, automated controller setup designed mainly to keep the firmware up to date without much intervention, this is a hassle. It looks something like this:

[email protected]:~$ sudo apt-get update
[sudo] password for robert: 
Hit:1 http://mirrors.linode.com/ubuntu bionic InRelease
Get:2 http://mirrors.linode.com/ubuntu bionic-updates InRelease [88.7 kB]          
Get:3 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]        
Get:4 http://mirrors.linode.com/ubuntu bionic-backports InRelease [74.6 kB]                                       
Ign:5 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 InRelease                                         
Hit:6 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release                                           
Get:7 https://dl.ubnt.com/unifi/debian stable InRelease [3,024 B]        
Reading package lists... Done                             
E: Repository 'https://dl.ubnt.com/unifi/debian stable InRelease' changed its 'Codename' value from 'unifi-5.12' to 'unifi-5.13'
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.

It’s an easy fix. Just tell apt-get to ignore the codename field:

[email protected]:~$ echo 'Acquire::AllowReleaseInfoChange::Codename "true";' | sudo tee    /etc/apt/apt.conf.d/99releaseinfochange
Acquire::AllowReleaseInfoChange::Codename "true";

and then it works!

[email protected]:~$ sudo apt-get update
Ign:1 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 InRelease
Hit:2 http://mirrors.linode.com/ubuntu bionic InRelease                                                           
Hit:3 http://mirrors.linode.com/ubuntu bionic-updates InRelease                                                   
Hit:4 http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release                                           
Hit:5 http://mirrors.linode.com/ubuntu bionic-backports InRelease                                                 
Hit:6 http://security.ubuntu.com/ubuntu bionic-security InRelease                                                 
Hit:7 https://dl.ubnt.com/unifi/debian stable InRelease                                                           
Reading package lists... Done