Hio. If you wish cork, and want to recycle an existing handle from another rod, you have 2 options. One is destructive to the rod, the other is destructive to the donor.
One is easy, the other is hard, and time consuming.
Easy/Destructive option: cut off the rod off just above the existing grip, add a pin to the rod, and splice on the cork grip, hoping that the bore diameters of the pin and the donor grip are close.
Hard and time consuming: remove the stripping guides off the butt section of the rod, carve off the synthetic grip, take a rat tail file, and file out the rod inside the donor grip, and slide the modified cork into position with a coat of epoxy to make the bond.
Re-wrap the stripping guides.