How extended must my extended paper be?
I don't think there's a single "default" answer here. However, I would suggest that if you are planning to publish in B's journal, that you do both kinds of extension: provide both additional explanations that were in the original material, plus present different results (if possible) than what was in conference proceeding A. You should also avoid direct repetition of material, where possible. 

However, before agreeing to this, I would also consider asking the journal editor to provide you some examples of what he or she considers "extended versions" of other proceedings, and review them to see what you think of the extensions. If you're not comfortable with the changes (in that they're not expansive enough to be a "new" paper), then you're free to tell the journal you're not interested.