Can I ask what you're basing these prices off of? Is it simply due to Radio Shack selling the S3 for the same price as Sprint last year or do you have some kind of insider info?
I'm not sure what you're basing this on, but my whole family is on Sero and we've been able to upgrade via the website for at least a year. My sister ordered an S4 over the summer via the website, and I did the same thing in early October for a Note 3 (later returned). When you begin the upgrade, it does say you'll need to change your plan, but as soon as you hit continue and get to the next page, it says "Good news, no need to upgrade your plan" and you can check out without changing anything. It's been like this quite some time and you not being aware of this makes me question the pricing you mentioned above or at least assume that it's nothing more than speculation.
PS - I hope you're right lol