This is copied from the tips and trick thread in Maya Chat:

If you're running multiple versions of Maya, then this tip is for you. This will enable an older version of Maya to open a newer version's file without doing some vodoo import export routine;
In the newer version, save the file in the .ma format. Open the .ma file in a text editor, preferrably one that supports rich text formatting. On Windows I recommend Wordpad vice notepad. In the editor change line 4 where it says Requires version "X" so that the value in quotes matches whatever version you need to open the file with. To be safe you should probably check the header info from an existing file for the target version. The image below shows two different header sections, the bold red show which areas need to match. In this example the 6.5 in read would be changed to 6.0.

Check out more cool stuff: https://forum.simplymaya.com/showthre...threadid=15467