I should google more

"Upgrades between LTS releases are not enabled by default until the first point release, 12.04.1, scheduled for July. It is recommended that most LTS users wait until then before upgrading to 12.04.

If you choose to upgrade before then, you can pass the -d option to the upgrade tool, running do-release-upgrade -d or update-manager -d, to upgrade from vanilla 10.04 to 12.04."