Reverting an object back to mirrored state (for symmetrical editing) after "APPLY" in Modifier panel was used I created an animal body using planes, extruded it, then mirrored the extrusion. 2. I then clicked "APPLY" in the modifier panel so that I could extrude the legs individually in different positions on either side of the mirrored halves. Everything is fine up to here
Now I need to get back to where I can edit with symmetry (like you can do right after a mirror) and I can't find how to do that.
Delete half of your mesh, add a mirror modifier. Now you can tweak your mesh and all changes will be replicated across the selected axis.