Your Organizer catalog should still be readable with your older versin of the Organizer catalog, Neale.
Converting the catalog just moves the old data into the version 9 Organizer so that you don't have to recreate it from scratch.
It probably depends on the Elements Organizer versions you are talking about. See http://kb2.adobe.com/cps/859/cpsid_85996.html.
Starting with PrE version 6, the format of the catalog changed from MS Access to SQLite. Once the catalog is converted (which actually a copy of the catalog is made and the original catalog is untouched), you will not be able to access the same catalog between the different Elements Organizer versions. It seems logical that you cannot 'share' a catalog beteewn Elements Organizer versions 9 and 5 for example.
A statement that makes me worried about even sharing catalogs from PrE 8 and PrE 9 comes from the above knowledge base article linked above "A catalog that has been converted to the Elements 9 Organizer format cannot be used by any previous version of the Elements Organizer. Because the original catalog remains unchanged, you can use it in the earlier version software."
If this statement was true, this means that you would not be able to see changes in PrE 8 that were done from PrE 9. I would be worried if a conversion is required on the catalog from PrE 8 to PrE 9.
I also wonder about the backwards and forwards compatability all of the other files such as the media cache files.