Looks like the seamless upgrade works just fine but none of my 1.5-era themes are usable. It looks like the theme-switcher is somehow to blame: if I simply rename my preferred theme to one of the two that come with the installation, I can see it.
<more fumbling . . .> it seems to work now, but not for all themes. Your mileage will certainly vary.
The detailed upgrade steps (ie, not for the impatient) are here. Little things like backing up your database are covered there.
I don’t even know what the alleged benefits are, other than 2 being a higher number than 1.5 đŸ˜‰ That’s always been enough for me.