I modified and existing rig and I want the left side to mirror the changes I did on the right side. Is there a way to do this without deleting half the character?
I assume you mean that you edited the Rig. In that case you can do this:
- In the tool shelf: Vertical Options Tab enable X-Mirror
- In your Armature, in Edit Mode: select all bones which you have modified (similar like in the image)
- press "g" followed by RETURN will instantly adjust the other side of your rig.
Note: This works only if the Rig's symmetry plane is YZ (character looks along Y axis) and when the Bones are named according to Blender's naming conventions, see http://wiki.blender.org/index.php/Doc:2.6/Manual/Rigging/Armatures/Editing/Properties
I think the closest you are going to get is Mesh > Snap to symmetry:
It works by snapping the nearest vertices to a mirrored position, however this won't always work perfectly if you have larger changes. It probably won't work at all if you have a lot of asymmetric topology.
If you have yet to make the changes (or you can make them again), you could try X mirror: