I've never found lean-forward to be an issue - at least not after I acquired the habit of walking erect. Like any other habit, it's something you have to consciously remember to do, until you do it automatically.

I have noticed that it's a bit easier to remember with an external frame pack, but not significantly so. I'm not sure why, but I find I'm more erect with my Exos pack than I am with my Kestrel pack. (Yes, the Exos is an external frame - the giveaway is the trampoline suspension. It's a very high-tech external, that is far more comfortable than my 1980s aluminum pipe pack, but it's still an external.)

In short, the pack only makes you lean forward if you let it. It's very easy to walk erect with any pack, especially with a load of 25 pounds or less (which is not difficult to attain with today's gear, unless you're on a longer trip where you're carrying a lot of food.)