Yes this is something i tryed to do too.
There are some script on the web but compatibility is equal to -300.
You could do something like this:
If, for instance, you start as a Vampire, then you can make a choice as Kest said. Then you'll make a choose class events, with the condition:
If you are a vampire, then you can change to "vampire warrior, vampire mage..." whatever.
Then, when you'll want again to change class, you'll simply put some conditions:
If you are: "vampire warrior, mage or vampire whatever", then, you can open the menu for switching one of your vampire classes, and exactly the same for werewolfs and the rest.
I think it would work perfectly, it's just a bit of events and doesn't make errors anywhere.
It's pretty simple.
It becomes more complicated if you have 4 characters and if you can switch them with other characters, then the menu would need to be scripted, i really wouldn't know how to build it on events and show choices.
As for the item requirments i must admit i don't understand anything of what you want, please write it again clearer.