Red Bull Racing have decided to change Max Verstappen's chassis after Free Practice 2. The reason for the switch is unknown up to this point, although it won't cause any kind of penalty for the Dutchman.
According to Dutch source GrandPrixRadio, there are three different possible causes for the switch of chassis. The chassis could have been damaged, there could be a fault in the wiring or there could be a rip in the fuel tank.
The same source reported the last of those three options is the most likely one, as Verstappen couldn't complete his entire program during FP2 because of it. The chassis has become more complex this season because of regulation changes with more things attached to it now, which is why the Bulls could have opted to just swap out the entire thing instead of one component to make things easier.