Here it is, I think the "pretty much the entire car" is an exaggeration, but it is obviously huge if not the chassis itself.
McLaren admit ‘entire car’ has been redesigned:
“Most of the upgrades we will see over the course of the following events, so there will be a major overhaul of the car that will be delivered across Austria, UK, and Hungary,” said Stella.
Explaining the thinking behind what will be delivered, Stella added: “Relatively soon at the start of the season, we realised the car needed a fundamental redesign.
“So this redesign actually involves, I would say, pretty much every single aerodynamic part. That’s why the upgrades will be spread over the course of a few races. We even had to redesign some parts under the bodywork.
“That’s also why it took some time to be in condition to deliver these upgrades. So I would say pretty much the entire car.”
https://www.speedcafe.com/2023/06/17/mc ... edesigned/