Do you want to upgrade from Ubuntu LTS to Ubuntu standard release?
That will do the trick:
# default behavior for the release upgrader
# default prompting behavior, valid options:
# never -- never prompt for a new distribution version
# normal -- prompt if a new version of the distribution is available
# lts -- prompt only if a LTS version of the distribution is available
…set prompt to normal (Prompt=normal), then "do-release-upgrade" and take care!