if its in mb, no can do. if in ma your good to go.
if in maya ascii then you need to go in with a text editor and change every place that it says maya 6 to 5. You still might have problems, error's, etc. But I have not seen many problems like this. Depending on the file though it can refuse to load it like this. You can thin always try exporting the objects you have the most work in to do this.