This is reproducible as well with 'apt-get dist-upgrade'; ubuntu-release-upgrader simply bails instead of letting a "wrong" upgrade path be calculated, but the removal of unity is reproducible with simple apt-get.
We can hint more strongly to apt that unity should be upgraded and unity-common should be removed by adding an additional Breaks: unity-common (<< 7.1.2) from another unity package lower in the stack - such as libunity9.
This is reproducible as well with 'apt-get dist-upgrade'; ubuntu- release- upgrader simply bails instead of letting a "wrong" upgrade path be calculated, but the removal of unity is reproducible with simple apt-get.
We can hint more strongly to apt that unity should be upgraded and unity-common should be removed by adding an additional Breaks: unity-common (<< 7.1.2) from another unity package lower in the stack - such as libunity9.