This is for changing the start date (and finish date, if you want) in both MS Project 2003 and MS Project 2007:
- Open your project document
- Go to the "Analysis" toolbar
- Click on "Adjust dates"
- Change the start date to your liking
Doesn't the above seem simple and straightforward to you?
Note that you can always change the start date of a project in MS project, regardless of how far into the development of your project schedule you currently are.