[nanny]

>OK, but I was talking about the file headers. :)

Gotcha, then you are correct. The MIT/Expat only requires: "The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software". As long as they are in compliance on that regard then they are in the clear.

>Sure, but the new work is the portions that you've changed, not the portions that you've copied, right?

No, the new work is the piece of software work as a whole, not the individual files. "Work" in this context is a legal term that includes all of the source code and nonliteral elements of the software, aka the Structure, Sequence, and Organization https://en.wikipedia.org/wiki/Structure,_sequence_and_organi...

[asdfasgasdgasdg]

R.e. your second statement, I can only presume that you're right, but I'm confused about how this works with copyright license agreements, or the cases where projects have had to go and get copyright releases from authors of individual lines of code to make a license change. If the copyright is on the entire work, how can a contributor of just one line of code own the copyright? Anyway. You seem more versed in this than I am, so I bow to your expertise.

[nanny]

In that case, there would be multiple authors of a single copyrighted work. I'm only familiar/knowledgeable with GNU copyright assignment, however, not copyrights in general. https://www.gnu.org/licenses/why-assign.en.html