In my old version of Sonar I could solo Reason 5 parts and render each to its own track.
 
How do I do it in these new versions?

why dont you just bounce the mixer tracks out of reason, unless you are wanting to render through sonar as 32fp files, you'd be as well just bouncing down 24 bit from reason, but its no different from wat you would do with any other version of sonar ???, highlight the audio track in sonar, where the midi files start and finish in reason and bounce to track !!! in track view in sonar you go to the tracks drop down list and choose to bounce from there

Were they connected via Rewire? If so then you. Need to be running both Reason and Sonar as 64 bit or both 32 bit. Rewire doesn't work unless both the slave and host programs are running at the same bit depth.