When you upgrade from 2026 to 2027, it leaves the HD 2026 installation intact and does a whole new installation of HD 2027. When you first launch 2027 it will take you through a migration guide, asking you what you would like to transfer from 2026 into 2027 (Projects, Libraries, etc.). This is as simple as checking boxes and clicking OK, and the program does the rest, but if you run into any issues you can contact tech support.
PhillipCA's post in "Upgrading" from HD 2026 to 2027 was marked as the answer