As a fellow Android 1.6 user (I'm on an HTC Magic), I can confirm that there is no easy and safe way to change the Google Account that your phone links to.
Anything you've saved into Google Contacts, Google Calendar, etc can be exported and imported into your other account fairly easy, other settings and data aren't so easy.
Factory resetting your phone is the only real option here, note that if you've bought any apps from the Market you are out of luck here. The Factory Data Reset option is under the SD Card & Phone Storage menu, and you really need to be careful about using it for the reasons above.