What happens on July 31?

The reformers say they've run out of patience and have set a deadline for moving to the new system.
At 8 p.m. Eastern time on July 31, they're threatening to stop recognizing transactions confirmed by miners who haven't adopted the upgrade. That would create enormous uncertainty in the bitcoin economy, as no one could really know if the bitcoin they'd just paid (or received) was actually moving through the system the way it's supposed to.
Some big bitcoin miners - like Chinese bitcoin mining equipment giant Bitmain - haven't signaled support for the new system. A rift could result in two or even more incompatible versions of bitcoin.