There’s some confusion over the UCI rules regarding bike changes. There’s nothing really new today for the cobbled stage. In short, wheel changes are allowed from support staff but for the whole bike, it has to come from the team car.
Here’s the rule in full detail from the UCI rule book:
2.3.029 Technical support
Riders may only receive technical support from the technical personnel of their team or from one of the neutral support cars or else from the broom wagon.
In the event of any change of bicycle during a race, the bicycle abandoned by the rider must in all cases be recovered either by vehicles accompanying the race, team vehicles, a neutral service vehicle or by the sag-wagon. Mechanical assistance at fixed locations on the course is limited to wheel changes only.
Any mechanical assistance which fails to meet the obligations above will result in the disqualification of the rider either immediately, or after the race if proven by any means and verified by the commissaires (article 12.1.001).