This article explains how the iPhone calendar and Yahoo! Calendar avoid overwriting each other.

Apple separates the where the information is stored which prevents data from overwriting each other. Before an iPhone user adds Yahoo! Calendar they will have a calendar on their phone already called "All on my iPhone" or "All from My PC".

Once you add the Yahoo! Calendar you will see a new set of layers called "Yahoo! Calendars". These will be separate and distinct layers. The events you had on your phone will not sync to the Yahoo! Calendar, events will only sync back to Yahoo! if you add them to the Yahoo! Calendars.