For me, this exposes a weakness in the mental model many coders seem to have about the operation of copyright.
Consider a pile of bricks, representing code contributions to a work. In one (surprisingly common) model, each brick is painted in a colour representing its licence status; red for BSD, blue for GPL, green for Apache, and so on. Whoever made and placed any given brick can consent to its repainting, but nobody else can, though anyone can remove a brick from the pile. In this model, a pile of Alice's bricks, painted blue, is added to with blue bricks from Bob. Alice now wishes to paint the pile red, but cannot because of Bob's bricks, so she asks Carol to make some red bricks with which to replace them. Once the pile is entirely composed of Alice's blue bricks and Carol's red bricks, Alice repaints her own bricks red. How can the sometime presence of Bob's blue bricks be a problem?
According to a practising barrister and sometime lecturer in copyright law with whom I have discussed this1 a better mental model is a pile of bricks under one or more comparably-coloured tarpaulins (waterproof covering sheets, aka tarps), with names written on them. Here, anyone can add or remove bricks, but any time you touch a pile under one or more tarps, you add your name to the list(s) written on the tarp(s). The colour of a tarp specifies certain rules: for example, bricks removed from under a blue tarp can't be used in any other pile which is not also under a blue tarp. You can in some cases combine piles under various tarps, and throw a new tarp over the whole lot, without necessarily removing the under-tarps. Certain operations on certain coloured tarps (eg, replacement of a blue tarp with a red tarp) requires the consent of everyone whose name is written on the tarp.
I do not mean to suggest everyone should adopt this mental model, and certainly not all the time, because like any abstraction it too has problems. But thinking about it may reveal to you when you're (quite possibly unintentionally) using the coloured-bricks model, because that is not a good model for copyright. For a start, coloured-bricks falls foul of the Ship of Theseus problem, as we have well-documented here. The tarp model has no issues with the Ship of Theseus, and that alone, to my mind, makes it useful.
The tarp model makes understanding this question less-problematic. Firstly, Alice piles up bricks under a blue tarp with Alice's name on it. Bob comes along, removes several, and adds his own name to the tarp. A replaces them. Carol then slides a small pile of bricks, under their own tiny red tarp, underneath the big blue tarp, removes some of the bricks replaced earlier by A, and adds "Carol" to the blue tarp. You now have a blue-tarp-covered pile with three names on the tarp: Alice, Bob, and Carol. The consent of all three will be required to replace the blue tarp with a red tarp (again, with all three names on it).
In short: Alice, Bob, and Carol are all rightsholders in the current work, either by virtue of their contributions to it, or by having been rightsholders in an earlier version, of which the current work is a copyright derivative. The consent of all will be required to relicense it away from GPL.
If Alice can apply Carol's changes to the version immediately prior to Bob's doing any work on it, and if Carol's changes were made without knowledge of anything B had done, then she will have forked the project to a point where Alice and Carol are the only rightsholders, and relicensing will be possible without Bob's consent.
1Nevertheless, I am an imperfect conduit, and any mistakes are of course mine.