The iPhone's information will never be replaced with what's on the computer unless you scroll down to the bottom of the Info tab and click the checkmark to replace it on the iPhone. By default the first sync with a new computer or new user should prompt you to merge information. After the first initial sync, the iPhone will do a bi-directional sync and merge information on the iPhone and the computer with each other. If you enable syncing of contacts, then click Apply in the lower right, it will begin syncing and ask you to confirm merging of the information which will consequently pull the said information to the computer.