Burn your upgrade. Use the profit made selling the S3 through Craigslist to make purchasing of your next phone out right affordable. Recognize that you will never use an "upgrade" again if you wish to retain unlimited data for years to come. If you go this route the most sensible thing would be to sell the phone you keep when it is valued around $200. My next phone purchase will probably be a slightly used phone off of craigslist. Purchasing a slightly used phones will have cost less and have no taxes.
For instance, a Galaxy Nexus can be purchased for around $350 used on craigslist now. Hopefully it takes nearly two years for the S3 to be worth $150, and getting a 6 month old nexus will only be $200 more. I will always be behind the curve with this method, but I don't mind. My fascinate flies with ICS, and that is nearly a two year single core device.