I NEVER have to thread my serger's bottom loopers - the ones that everyone hates to thread, and I have had it for more than 20 years. When I want to change colors, I cut the looper thread about two inches above the tension disks. This is just below the thread guides. Take off the old thread cones and add the new ones. Put the thread through the guides until you come to the cut end that is still in the machine. TIE THESE ENDS TOGETHER SECURELY.
For the top threads, I remove the spool and pull out all the thread. The top threads are very easy to get through the needle and put under the foot.
Then just sew until your new looper thread is coming out under the foot.
Well, once in a while the knots come loose, and I have to rethread the bottom, too, but a few times in a few years doesn't bother me.