The manual tells you not to just pull it out, but... you slowly pry the existing one off and pull it out, then work the new one into place and it will get pinched to hold in place.

If you're trying to go smaller, have at it.

If you're trying to go bigger, do yourself a favor and just get a tennis racket or baseball bat grip wrap instead of messing with the grip pad. Of all the radios I've used through the years, the Sanwa has the least "sweaty grip", and a cheap baseball bat wrap (one pack will do your radio twice, easily) was a simple way for light customization, and limit the "sweaty grip" shifting.