Would you like to upgrade from Ubuntu LTS to Ubuntu standard release?
The below command will do the trick:
# default behavior for the release upgrader
[DEFAULT] # 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!