Please clarify what you are referring to as the "Map Legend Widget".
Ben,
I think the live maps widget is one of Roberts it's now called the table of contents/legend widget is has been updated to 3.1:
http://www.arcgis.com/home/item.html?id=e2cb71d5c04d40d19e9f945ae0db7ce8
Regards
Anthony
Ben,
It depends on if you are using compiled or uncompiled code. If compiled then no you will have to upgrade. If you have the source code then bring it into your application, work through any errors then re-publish it may work
Regards
Anthony