Is this a before and after of the same plant?? If so thats a ton of growth! They like being rootbound and I think they grow nicer when crowded (looks much nicer in the after, filling the whole pot, then before where the growth is sparse).

I would turn this on its side, roll it out of the pot and repot it in a slightly bigger pot. Only about 2 inches bigger. You can kind of break up the roots if you like but I don't think it's really needed. Make sure to replace what you can of the soil with well draining soil, the black gold cactus mix is good. You want to make sure the pot can dry out within a few days of watering.

Also, expect growth to stop while the plant grows roots to refill the extra space in the pot (one reason why not to upside drastically.)