Hi Vladimir,
Please find attached a sample extensibility behavior that I have created to demonstrate how you can change the default symbol label for Class Breaks and Unique Value renderer (see jpg attached), plus how you can also get hold of the ImageSources for each LegendItem in the Legend control. The behavior that you will see in LegendEventBehavior.cs.txt shows how to get hold of the legend control itself once Map Contents control is loaded, and how to subscribe to Legent.Refreshed event and overwrite the label value. The couple of commented out lines show how to get to the Image Source which you can use to generate better print-outs.
If you want to add your behavior to be part of your default Viewer Application for Silverlight Viewer, you can just modify the Behaviors.xml file to include your new LegendEventBehavior (see Behavior.xml.txt). However, for SharePoint we haven't exposed the Behaviors.xml file, so you will not be able to have your behavior be part of all new Map Web Part that get created, but it will need to be added explicitly to each Map Web Part using the SharePoint Ribbon (ArcGIS Map Web Part -> Application -> Behaviors) once the map web part gets added.
Hope this helps!
Regards,
Elitsa Baklova