That's exactly what happened to Samba. It was originally licensed with "GPLv2 or later", and now it's "GPLv3 or later". The main contributors to Samba were leading the switch, but they didn't get explicit permission from every single copyright holder, nor did they have to.
They didn't need to contact the contributors because of that "or later" clause. The license explicitly lets anyone relicense to GPLv3 without asking anybody first.
Linux's license doesn't have that clause, so you're not automatically allowed to relicense. Switching to GPLv3 would require the copyright holders to explicitly do so. Since Linux doesn't require contributors to assign copyright to some central organization, "the copyright holders" means all contributors, past and present.
