That's right. And in airline pricing land (which has traditions and terminology established long before frequent flyer mileage became particularly relevant) a fare upgrade can quite happily take place within a cabin. So I->R would be an upgrade, as would I->A.

The idea in this case is that any upgrade would allow passengers to change the fare (since I is the only public fare which has no changes permitted) and so negate the purpose of the original fare rules under which, of course, changes are not permitted.