I've tested an upgrade from Wily to Xenial using the version of the ubuntu-release-upgrader from -proposed and with apt version 1.0.10.2ubuntu1 installed, with that version of apt the release upgrade process would not start. Once I installed the newer version of apt, I was able to start the upgrade.
I've tested an upgrade from Wily to Xenial using the version of the ubuntu- release- upgrader from -proposed and with apt version 1.0.10.2ubuntu1 installed, with that version of apt the release upgrade process would not start. Once I installed the newer version of apt, I was able to start the upgrade.