pwrusr.com

Do you want to upgrade from Ubuntu LTS to Ubuntu standard release?

That will do the trick:

vi /etc/update-manager/release-upgrades

# 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
Prompt=lts

…set prompt to normal (Prompt=normal), then "do-release-upgrade" and take care!