The only supported (=tested) upgrade from non-LTS to LTS is from the last version before the LTS, and you should not change the sources yourself when you do tha

The only supported (=tested) upgrade from non-LTS to LTS is from the last version before the LTS, and you should not change the sources yourself when you do that. When you skip releases, no distro will officially support such upgrades (except for Ubuntu LTS-to-LTS, without skipping an LTS), and things might break. Also, when you really want to skip releases, it is often better (IME) to change the sources manually and not use do-release-upgrade, but use aptitude and/or synaptic and/or other such tools to fix the dependencies manually. In any case, you might have to fix some breakage afterwards (or mid-upgrade…), as it is untested & unsupported.