My understanding is that going backwards to 1.4 is not supported, and I can't blame them for not supporting it. Use the lowest common denominator (i.e. 1.4) if collaborating, or if you desire to publish in the Marketplace, or whatever, and test in 2.0 by importing 1.4 projects. Fortunately 2.0 includes the license to use 1.4 as well, so it's there if needed. But don't expect an easy migration path backwards.