|I was recently faced with undoing some LONG seams that I had serged. After sweating about this for a little, I thought of an easy way to do it. It is easier if you haven't finished the ends, but can be done regardless.
With your seam-ripper, carefully pick out the needle thread(s) from the end of the seam. Note that they will be more accessible from one side than the other. When you have enough to grip, pull the thread out of of your material, as you would when pulling out basting stitches. If the seam is long, cut the needle threads part-way to make it easier. Once this is done, the looper threads easily pull out.
update: I've found in some fabrics the threads don't pull easily. In those cases, I've taken to creating an opening in the seam from the inside, then from the right side I can get to to needle thread(s) and pick them out without simply cutting and leaving lots of thread bits. The looper threads just fall off afterwards. This is slower than being able to pull out the needle thread, but still faster than other methods I've tried.
And yes, I do end up ripping my serged seams that often. :o