See also Upgrade to Byzantium
Yes, Ubuntu version upgrades do that automatically i.e. disable all extra repos before the upgrade. After the upgrade it does not then re-enable any extra repos. I guess the thinking is that
- it is up to you to work out whether your specific extra software is compatible with the new version of Ubuntu and, if so, which version of that extra software you might need
- it is important for fault isolation to establish a working baseline i.e. the system worked immediately after the version upgrade and only fell over once you started polluting the system with extra repos
