Just go to the "My Ebay" page and click the "Accounts" tab. Down near the bottom of the page in the "Account-Related Links" area click the "Update my personal information" link - then in the next window click the "Change my email address" link. This does not change your user-id - just the email address that eBay uses to contact you. As long as you don't change your user-id all the feedback and everything stays the same.

Chas