I change all of it, but I take him out and put him in a tiny bowl with his old water first, and I actually keep bottles of pre-conditioned water in my closet to change it out quickly and with the temperature already adjusted. I put two drops of the conditioner per the 1L water bottle. I wash his tank, his rocks, fake plant with regular tap water. I usually check the levels of everything with a fish tank test strip.
One thing I do notice is the water in my house is really hard, and I don't know why the conditioner doesn't make it much better.
But anyway, then I put him in his new water right after adding everything (but everything is the same temperature).