Folks are seeming to fail to remember, or know at all, what the differences between S1 and S2 are. As released versions, there were huge differences. New content added to the old content. Both a physics and graphics update. New features such as tire wear and heat and pitstops. Now think of it as the differences between S1 and S2 licensing? What do the S2 users have that the S1 users don't have? Content and only content. They still got the physics, graphics, and feature changes.
I'm speculating that this will be the same with S3. The S1 and S2 licensed users will get any and all physics and graphical improvements, causing no problems with compatibility. They just won't get the content. By content, I am also specualting that stuff like rain, wet track, debri and/or dirt on the track, etc will be like the content that Aston, Kyoto, GTRs, LFRs were to S1 vs S2.
They won't be backwards compatible simply because there will be no backwards. Just as when S2 was released, the lower licensed users will have access to the same physics/graphics/other that the S3 users will, except for the content.
The LFS devs are the ones who will make the current content compatible with the new version. The rFactor devs have nothing to do with the content in their sim. Thus, nothing will be compatible.