Distribution upgrade using the so-called manual or brute force way.
sudo apt-get update; sudo apt-get dist-upgrade sudo sed 's/utopic/vivid/g' -i /etc/apt/sources.list sudo apt-get update; sudo apt-get dist-upgrade
Off course, the distro upgrade would not be that smooth as there are many legacy repository that you'll need to remove or disable in in either the /etc/apt/sources.list file or /etc/apt/sources.list.d folder.
Using the Ubuntu source list generator, below is the most minimal repository URLs after some trial and errors as some of the repository components (Restricted and Multiverse) does not exists. Hence, only main component is used.
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to # newer versions of the distribution. deb http://download.nus.edu.sg/mirror/ubuntu/ vivid main universe deb http://download.nus.edu.sg/mirror/ubuntu/ vivid-updates main universe deb http://download.nus.edu.sg/mirror/ubuntu/ vivid-security main universe deb http://download.nus.edu.sg/mirror/ubuntu/ vivid-proposed main universe
Note that we're also include those from pre-release updates (vivid-proposed) since I opted to get the most latest bleeding edge packages. If you want manually update those pre-release updates, you can set a different priority to this repository.
Make sure this file /etc/apt/preferences.d/proposed-updates exists with this content:
Package: * Pin: release a=vivid-proposed Pin-Priority: 400
And run this command to selectively select those packages to be upgraded from vivid-proposed repository.
$ sudo aptitude -t vivid-proposed