Pleaseee HELppp Meee!! DO YOU HAVE TO RETURN YOUR IPHONE 3GS OR3G IF YOU UPGRADE TO IPHONE 4?

hI iM JUST WANT TO KNOW IF i HAVE TO RETURN MY iphone 3gs to apple in order for me to be able to upgrade to an iphone 4..im not done iwth my contract yet and i have abouta year and a half...but when i upgrade do i have to return it back to apple...because i want to sell my iphone 3gs on ebay