Classic. I too was afraid to try the version 8. Generally, Mac will imbed a sort of INI-file on their latest release which prevents you from going backward. However, these System 9.2 to X.x issues have mostly disappeared, as the .dev community became 100% UNIX-like in their thinking. Now, you can run multiple [but only OS-X native] Ap-versions at the same time.

Dropping the (good) library entries, or file onto the <NEW> iTunes open window will merge the two libraries, although it's not a 100% trouble-free solution. You still have to 'authorize' one or the other before your weekend of pain is through.