First, you psych yourself up to endure some pain. True! Then I let it dry beyond its good health so the soil would stay intact when I dumped on a piece of old carpet. Then I used that carpet to wrap up over the spines and lift it up and set the root ball into the already prepared hole in the new container. Mostly successful - only 3-4 bloody puncture wounds. It got put off for a long time before I was psyched up enough. But the cacti responded quickly to its new and improved circumstances. I'd let it go so long the lower spines and lower part of the barrel were kind of flat where it grew against the side of the old container. I thought that would be permanent damage, but it recovered fast and fully. I can't tell where it was now.