@isolated-thinker If your OPO was on 15.04 the only way at the moment to get 16.04 is to use the installer and choose 16.04 dev from the options list and choose the wipe option for the best install. Yes all your info etc will go, so you will need to take a back up of your device first.
You have currently updated to the dev version of 15.04 from what you say. You can stay where you are on 15.04dev if you are happy with it and you will at somepoint in the future be able to update to 16.04 directly. However the time scale for that is not set in stone at present and is at least a few weeks away.
D