Elon has been hinting at faster-than-gas recharging rates for the past few weeks on Twitter. The Supercharger network upgrades are part 4 of a 5 part trilogy they've been running since April. It's widely rumored that part 5 will be battery swapping or something similar to enable ultra fast charging.
My guess: Superchargers will remain free as promised and the battery swapping will offer faster charging, but at a cost.