If tied properly your d-loop will not move up or down without a whole lot of effort on your part. Can it be moved after you tie it in? Yes, but you'll have to work with it quite a bit to loosen your knots.
You can tweak you D loops position by spinning it in 360 degree rotations around your string on theserving.
Look which way your serving was put on and twist the D loopin the appropriate direction to move it up or down. The serving twisted around the string will work much like threads (read nut and bolt)
However with tied in nock sets in place this will not work. Also, I would rather just retie a new one than screw around moving one over a 1/8" though.
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