I contacted support to seek a solution to a project that was done in stereo (by accident) that I wished to change to 5.1. After receiving instructions that did not completely solve the problem I figured it out on my own and I'll share here.
1) Change project properties setting for audio to desired setting (in my case to 5.1)
2) When I set the project to 5.1 it created the sound tracks req'd for 5.1 audio (front, center, etc). I copied the stereo audio track to each of the new 4 audio tracks, selected channel 2 for each track (which was the AC3/5.1 mix) and assigned the channel assignments as is when a normal import is done. Channels 1&2, channel 3, channel 4, channels 5&6.
3) deleted stereo track
Hope this helps somebody out.
OP
I'll post this to the forum for others to learn from.