According to Gene Munster , an analyst with Piper Jaffray, sales of  iPhone 4S  will go very well and  provides  that a good portion will be purchased by customers who currently have a  3G iPhone . Munster estimated to be approximately 18.8 million people from the ' iPhone 3Gpass all ' iPhone 4S , which corresponds to about 18 percent of total sales. There are about 27.6 million owners of iPhone 3G smartphones ready to change and 25% of these will choose the ' iPhone 4  and 15% will switch to Android .  Source