I think you can download a free trial of AECS3 from Adobe’s site and use it for 30 days… that would help you for this project at least, you’ll still have to upgrade eventually. I once saw a tutorial that showed how to open an AE7 project in AE6.5 by modifying the aep file in a text editor. I think there are just too many new features like Layer Styles, Shape Layers, etc for you to do the same with a CS3 project. AE7 wouldn’t know what to do with those new features.