VS2012 projects are mostly* backwards compatible with VS2010 SP1. The projects will undergo a conversion/migration process like in all previous VS versions, but this one is friendly towards VS2010. You should be able to make changes to your projects/references/code files and open them again in VS2010 without issues.
I say mostly because there are some features new in VS2012 that may not be backwards compatible. The most obvious, if you upgrade a project to .NET 4.5, it will no longer work in VS2010. See http://blogs.msdn.com/b/visualstudio/archive/2012/03/28/round-tripping-with-visual-studio-11.aspx for more information about this scenario.