Incompatibility of IntelliJ Idea 12.1 with projects created with previous versions

I've just upgraded to IntelliJ Idea Ultimate 12.1 and I see that there is an incompatibility with projects created with previous versions and when you try to open a project created with a previous version , the ide wants to convert it to the new format.
Do you want to comment on this change?
I mean what are the differences between the new project and an old one?
Thank you.