When you upgrade an old eLicenser-based Cubase licence (version 11 or earlier) to a Steinberg Licensing version of Cubase (version 12 onwards), the eLicenser licence is upgraded to a Cubase 11 licence that cannot be upgraded again. This serves two purposes: it stops you from upgrading the same Cubase licence twice, also it allows you to use all the old eLicenser versions of Cubase if you need to do so for compatibility reasons.
Thanks David!