The Organizers in v7 and v8 are different. You can convert an old version catalog to a newer but I doubt you can do it the other way.
Unless you changed it the catalogs are in "C:\Program Data\Adobe". And for v9 and v10 in a further subfolder "Elements Organizer". I'm not sure about v8 as I never used it.
So, if you want to live dangerously, BACKUP those catalogs first, then copy them and try opening the copy from the PRE7 organizer and see if it offers you a conversion option. Even if it works the v7 and v8 catalogs will remain independent - they do not share a common catalog.
Insanity is hereditary, you get it from your children