yet again I needed to download and verify a @QubesOS image, and yet again having the QubesOS official t-shirt (which has the Qubes Master Signing Key fingerprint printed on it) turned out to be very useful, actually.
@rysiek@QubesOS rotating the signing key will get a bit involved at some stage. "Please download this PDF and print it on an iron-on carrier. Cut to size, and place over the expired key, cover with a clean cotton cloth. Heat iron to III and apply equal pressure in smooth moves. Do not overheat."