It looks like they haven't been glued on so should just slide off. Sliding off can be assisted by inserting the long plastic pipe on a can of WD40 up the end of the grip. You then progressively move it around the circumference of the grip while giving a small burst of lube, at each rotation push the pipe in further. If it has been glued cutting it off is sometimes the only way to remove it. As far as fitting the new grips I've always used the wife's hairspray. Spray the inside of the grip with the hairspray which will act as a lubricant and allow you to slide it on easily. Once dry it will act as a adhesive. It doesn't seem to matter what brand hairspray the wife has used over the years they all seem to work, however she is currently using Schwarzkopf EC strong styling. :hehehe To remove use the WD40 method.
Regards Martin.