I removed the molded plastic grips from my BD cheapy collapsing poles by immersing the handle in a pot of boiling water for a few minutes. With a leather gloved hand I was able to pull them right off.
As far as replacement goes, lots of folks like to use fishing pole grips. I wrapped mine with cork bike handle bar grip tape. After wrapping I ran a bead of supper glue around the wrap to keep it all together. Several hundred miles later they are going strong.