I have the RS Convertible Backpack. As others said, it has a hip belt and expands about an inch (which makes it too big to be a carryon). I think those are the main differences from the Classic.
I have used this backpack for several trips over the past few years, and I’ve decided to move on to a different backpack. It’s a good size and very durable, etc, but I don’t think it’s very comfortable overall. The hip belt helps a lot (taking the pressure off the shoulders), but after our last trip I vowed to either find a more comfortable backpack (sorry Rick!) or switch to a wheeled bag.
So last month I purchased an Osprey Fairview. I have high hopes for it, although I haven’t used it on a trip yet. I did try it out at REI (with their backpack weights) and it seemed a lot more comfortable and ergonomic. It has a hip belt, sternum straps, and is adjustable to your torso size. It also has load lifter straps and the shoulder straps have a lot more padding than the RS Bag. The Fairview is sized more for women, and I think the mens/unisex version is called the Fairpoint. I got it on sale for around $130. It’s more ‘hiker’ looking that’s some of the other options out there, but I don’t mind!
I don’t regret having bought the RS backpack, because it was inexpensive (under $100 on sale at the time) and I learned that I like traveling with a backpack vs a wheeled bag. I probably wouldn’t have wanted to spend a lot more than that not knowing whether or not I’d like backpack style luggage. I just think it’s time for an upgrade.
My husband also upgraded his RS backpack and he went with the Cotopaxi Alpa. That was my second choice, in fact I had a hard time making the decision between it and the Osprey. The Cotopaxi has more organization and it opens like a suitcase. It’s also very cool looking and not so hiker-ish. I learned during our search that there are now TONS of luggage style backpacks available at many different price points.