FWIW, I have opened considerable numbers of Logic 5 PC files with Logic 8 and 9. It always worked, apart from losing settings in VST Plugins, although in many case the corresponding Audio Unit was actually recognized and loaded. The other thing to watch out for is whether the old files were created with Universal Track Mode on or off. If it was off in 5, you need to switch off UTM in Logic 9 before loading the file. If UTM was on in 5, it needs to be on in 9 (which is the default).
Files from Logic versions older than 5 need to first be opened in Logic 5, 6 or 7, then saved, after which they can be opened in Logic 8 or 9.
kind regards
Mark