Since our object was originally created symmetrical, we can use \C020202Smart Resymmetry\C484848 to mirror the deformation changes on one side of the object.

Normally, \C020202Smart Resymmetry\C484848 averages the displacement differences on both sides. We can 'protect' one side and force the other side to match, by masking the 'good' side.

We orient the object so it's facing us, then CTRL-drag a rectangle over the portion we wish to protect. It appears darker to indicate it's masked. With one side masked, we choose the appropriate X, Y or Z axis and press \C020202Smart Resym\C484848.