Do not alter the harness, just replace the connectors. Buy either just replacement bodies, or pigtails and de-pin the pigtails. Chopping up perfectly good and undamaged harness just adds failure points.
You will slide the lock tab on the inside out of the way, then pull it through the hole in the side of the connector. Then reach through with a de-pinning tool and release the terminal lock/retainer on the inside of the connector body. Typically one can buy an assortment of depinning tools, like a pack of 60 on various rings, and about 10% will be capable of releasing the terminal locks. I find it easiest to just take an old connector I'm replacing, crush it with some pliers, and that will give you an idea of the internal design of the terminal locking system. Works nice when you already have broken connectors that need removed anyways.
