If they are glued on, 3M makes an "adhesive release agent" that you can get from auto body shop suppliers. You'll want to protect your bike from any drips that may get on your paint. I don't remember if it will damage paint, so I would read the label carefully. I do know it works well on emblems, pinstripe tape, contact adhesives, and so on. Nice lookin grips, though. jimsjinx
I replaced mine about two months ago. When I pulled or twisted, nothing would let go. Then I used a thin screw driver in between the handle grips and the switch assy and carefully pried it. They came off very easy.
They are something like a chinese puzzle, you can't pull from the end.
Heating the grips a bit will help and judicious prying as already mentioned will often get them moving. The allen screws in the end don't hold them on, they're there to attach a throttle boss and to allow changing the color rings on the ends. Be careful prying on the throttle side so as not to damage the throttle sleeve.
Those particular grips look to be Kuryakyn Iso Grips which do ship with adhesive. The alcohol (above) works more as a low-residue lubricant than a solvent, so if you find yours to be glued tight, maybe something a bit stronger in the syringe?
I have been told by several people that they are glued on. I was told to use a very small diameter screw driver and slide it in between the grip and the handle bar. Then take some spray carb cleaner with the little red tube attached and slide the tube in where you inserted the screw driver. Being very carful and after covering any part of the bike that might get sprayed or dripped on. Give little sprays of cleaner and work the grip with a twisting action to loosen it. I have also been told that some times you have to spray then reinsert the srew driver and work it side to side then spray again and so on till the grip will slide off.
I have not had to replace mine yet so can't say from first hand experience but I hope this helps.