It's usually cheaper to change once you've begun travel because they do not reprice the entire ticket. Unless of course there's nothing but Y/B left, in which case you're going to upfare signicantly. So part of your calculus has to be fare class availability. How's it look?
Apart from that, to me the upgrade availability would be the main issue. That's going to be heavily dependent on the specific itinerary (and your willingness possibly to change routings to snag R). So there's not going to be one "correct" answer here in the abstract.