which is a so-called custom “payload” (additional element) of a coreboot firmware.
It is technically possible to flash another firmware (including coreboot with some elements just to boot your system) but it is probably not worth the hassle unless you really know the internals.
What I would do is just to sign the Qubes installation again after the update and forget about that Heads thing completely.
@saper Thank you very much for your reply. Much appreciated. It looks as if I have some searching to do, as I don’t know how to “sign the Qubes installation”, but I am grateful that you have put me on a path to a solution!