SDC can not be used on non-UA operated flights but you "can" change away from a non-UA flight to UA flight(s). But not all agents will tackle this significant of a change and it will not show up online or at the checkin kisoks.

http://www.flyertalk.com/forum/unite...-wikipost.html (see wiki portion)

However, you need to consider the risk if the SDC is not available. It is far from a certain especially if connecting flights are needed.