That is not true. The people who contributed the original Shunway code own the copyright to it. They are choosing to license it under Apache. You cannot change that since you do not own the copyright.

They may choose to "relicense" it under GPL if all the contributed agree.

What you suggest cannot legally be done. However, Apache is GPL compatible, so it can still be used in a GPL project, though its license will not change.

IANAL

It's shorthand for an end result that is in practice similar. If I fork the project, add new features, and license my new code under the GPL, a user of the forked version needs to abide by the copyleft provisions. They could, of course, continue to use the original Apache v2 code without my GPL'd additions, on a non-copyleft basis, since I cannot relicense the existing code.

You can make a derivative that also includes GPL parts, and the entire package will be GPL for practical purposes.

Yeah, but you can do a simple change and suddenly it's derivative and you can licence it as GPL.