As easy as any upgrade can be. And as always, you should NEVER uninstall previous versions of SONAR or risk losing plugins and instruments that were released with different versions. They take up very little hard drive real-estate, and you most likely lose some of your favorite "Cakewalk" plugins that came specific to that version.
For instance, my Sonar installations go back as far as 8.5 because that's when I installed Dimension Pro and Rapture and all the expansion packs libraries.
And while I don't use Dimension Pro or Rapture anymore ALL of these expansion packs of samples and synth presets still work and are listed in Rapture Pro.
Rapture Pro is essentially the old school Dimension Pro and Rapture synths married together into one Killa arse powerful user interface. And leaving old Dimension Pro and Rapture installations where they are can save you many hours of work having to reinstall all the expansion packs that make Rapture Pro and Session versions offer 10x to 100x more choices, i.e. MUCH more powerful.
My acronym for Cakewalk by Bandlab is simply CbB
Of course depending on how older your version of SONAR is/was, you may feel like a stranger in a strange land.
I remember feeling like that when the Skylight interface was introduced.. HATED IT! I stayed with version 8.5 Producer as my main whip thru 2 versions until I finally got used to it with version X3 Producer.
It took a while for it to sink in to the point where I realized how awesome the new changes effects my workflow...….. OMG...… This IS so much easier, it DOES make things much better, DUUUUUUHHHHHHHH.
Old dogs that refuse to learn new tricks just get angry and shrivel up before their time..