Disassemble the old backshell and strain relief.
Destroy the old “wire clamp” portion of the back shell by severing it with a pair of dykes. (it is hanging loose to the left) That will give some working room on the old harness.
Extract and move one wire, or one pair at a time to the new back shell.
Add strain relief and tie out.
The right side is more difficult, because there is no proper service loop in the harness.
WHY is it helpful to perform this operation on the RH side? There is no fit and clearance problem on the right. What a waste of time.