|Here's a handy way to rip out a serged seam without losing your seam allowance: with a small sharp scissors clip the very edge of the stitches where the upper and lower loopers lock on the edge of the fabric. It's OK if you get a tiny bit of the fabric along with it if you must. Find the needle thread, using a straight pin to help you pull out one stitch. Pull sharply on the needle thread and the whole thing will pull out if you have clipped all of the looped edge. All that's left after that is to pull out all the little eyelash length threads that are left in the fabric, and your seam is now undone.
The edge may look slightly chewed, but at least you haven't cut off your seam allowance in the process.