If you're on a Mac, it's not going to work 100%, but on a PC, you need to save as maya ascii (.ma) and then open the .ma file in a text editor (wordpad should be fine).  Now replace all the 7.0's with 6.5 and save the file.  This should be enough to make it open in 6.5.\n\nHowever, if there are any dynamics or character animation scripts being called or anything that might be 7.0 specific there still could be a problem.  For that you may need further assistance.  Ask another question here if you hit another wall.
how can I make a maya 7 file readable in maya 6?