Go into Media Composer. In your Master window, with the tabs for settings, project, format, info, etc, click on format. After doing so you can see there are two options. Project Type and Raster Dimension. Go to project type and you should have a few options. In this case there is 1080p/24 and 24p NTSC. This is the sample project from Avid that came with installing the 3.5 Media Composer software. I can downgrade to SD from HD and go back. These settings are all different from how you create your project in the start. For example, if I choose 1080i/59.94 I can go to 720p/ 29.97 or 30i. The raster dimensions also change. I can choose 1920x 1080, 1440 x 1080 or 1280 x 1080 depending on my pixel aspect ratios.

Sadly, I am unable to tell you how 4.0 works though, with different frame rates and formats. We will have to see if I can upgrade to that soon enough If there are any other issues in creating your projects or changing the formats, please post a comment and I will do my best to get your problem resolved.