Changing from stereo to 5.1 half-way through a project?

Hello,
I started editing a home video and foolishly forgot that our camera supports 5.1 until I had nearly finished editing! Changing the settings in the 'Audio' tab to 5.1 from stereo in the properties doesn't seem to change existing clips in the timeline, only new clips.

Is there anything I can do to fix this? I really don't want to start the whole thing again...

Thanks in advance.