When you try to open VI saved with later version of LabVIEW, you get this error :
But when you open VIs in a given version with a later version of LabVIEW, you have no information and that could cause big problems.
For many reasons I have to work with different versions of LabVIEW (right now I have 8.6, 2010,2011 installed at the same time), and it's a major problem when I open a project and by mistake choose the wrong version of LabVIEW.
Someone proposed to display a saving selection popup Here
But when you have already made some modifications, it's enoying to discover that you won't be able to save it.
I propose instead to display a message when opening a VI, and let the user chosse either he want to upgrade is VI version or not.
At least he can see his mistake and decide to open the VI with the good version of LabVIEW.