When loading a song created in a version of Logic Pro prior to version 7, the song format will be converted to the version 7 format. The first time this song is saved, a Save dialog will appear giving you the option to rename the file so that the old song file will not be automatically overwritten.
Note: Once saved in version 7 format, a song can no longer be opened with versions prior to version 7 (exception: Logic Pro versions 6.4.3 and higher can open the song, but without any Logic Pro 7 specific features).