I have done multiple SDC on a First Class fare, where one of my new segments was confirmed as economy and the other as First. Sometimes they've technically rebooked one segment as W for Comfort+. When they do this, it has put me at the top of the upgrade list, and I did clear as an upgrade once. Other times First remained full on that segment.

E.g. booked was Z on two segments. SDC was to W on first segment, Z on second. Cleared upgrade to OU on first segment.

How it credits to SkyMiles after you fly is very much YMMV.