Maya has really never been backwards compatible. If you work with a model/etc... in Maya 7 and then bring the file into Maya 8 the file becomes coded for that particular release.

I know about this because this happened to me when my school converted from Maya 5 to Maya 6. I had brought over one of my files to use Maya 6 but when I took the same file back to Maya 5, I got the famous script error.

Unless you have a backup file saved, you're gonna have to use Maya 8.