|I did not see this written out explicitly anywhere, so I thought I'd write this tip for the new owners of sergers:
When you want to change colors, don't take out the old and start all over again with the new (unless it wasn't threaded correctly to start!). Without loosening tensions, cut the old threads between the spools and the first thread guides. Knot the new thread ends to the old thread ends. I use square knots so they won't easily unknot. Set all tensions to zero. Raise the presser foot. Gently, gently, gently pull through the threads one at a time, in the correct order (upper, lower, right needle, left needle), making sure each thread is going into the guides correctly. Hopefully, you won't have to rethread everything, especially the dreaded lower looper, and if the knots let go before you get the threads all the way through, you may only have one or two guides to thread. The knots usually won't go through the needle eyes. Once you get the threads to the presser foot, cut them so the knots and the old thread tail can be pulled toward the back of the machine and get it out of the way. Thread the needles and set the new threads under the presser foot, making sure the lower looper thread is lying over the upper looper thread and none of the threads are tangled. Tighten your tensions to medium, then slowly turn the handwheel to get the serged thread tail started. If it looks okay, sew a little using your foot pedal to be sure it is not tangling up, and you are set to go. Just test the tensions, stitch length, differential feed on your new fabric, and serge.