I created a base mesh to sculpt upon using a mirror modifier, but then applied the mirror before adding a multires to sculpt with. During sculpting I went into edit mode and made some changes to the base mesh, forgetting that my alterations were not being mirrored. I tried re-adding the mirror, but it can't be moved above the multires. I've also tried manually correcting the problem, but it's still not how it should be. Is there a way I can make my base mesh symmetrical again, or do I just have to live with my mistake?
It is possible to make a mesh symmetrical without a Mirror Modifier.
Simply set your Pivot Point to be your 3D Cursor, place the 3D Cursor at the center of symmetry, duplicate the geometry to be mirrored (ShiftD), and scale (S) -1 on the axis of symmetry (in this case the X axis X).
You can set the Pivot Point to the 3D Cursor here:
Take this example of half a monkey:
Finally, pressing W and choosing Remove Doubles can get rid of overlapping vertices and weld the mesh halves together.
You'll want to Recalculate Normals after doing this (CtrlN) because the face normals get flipped inside-out in the scaling process.