Also a script to export projects from CS3+ was previewed in AE Scripts previews upcoming releases, and later released as pt_OpenSesame by Paul Tuersley.
You can Save and back up projects in After Effects CS5.5 and After Effects CS6 to previous versions. For more on the native feature, see Saving Backward for After Effects CS5 by Todd Kopriva and AE CS5.5: Save Project as After Effects CS5 by Chris and Trish Meyer.
Maltaannon has a short video tutorial demonstrating how to convert an After Effects project from CS4 down to CS3.
Jonas at General Specialist discussed the same topic in 2007 in Opening After Effects Projects in an Earlier Version.
Maltaannon uses his Trapcode Form oriented tutorial DNA Chain as the example. Jerzy adds:
"The trick is that you can copy-and-paste some elements from CS4 to CS3. It’s easier with After Effects’ native plugins like Fast Blur or Ramp, but it also works with 3rd party plugins as well to some extent. The main limitation I’ve stumbled upon is that you have to copy each section (or Group) of parameters separately. The easiest way to do this is to select a layer, press UU to reveal all modified properties and then copy paste them in groups. For example: Trapcode Form has several groups like Particles, Layer Maps, Visibility and so on and so on. You have to copy each group on it’s own. That also goes for the sub-groups.
As a side note I am working on a ready-to-use solution for this problem that will allow you to simply import your CS4 projects to CS3 or even earlier versions of After Effects. But until that day comes all you can do is follow the tips in this tutorial."