Note that I just encountered the following issue:
https://github.com/fedora-silverblue/issue-tracker/issues/536
Which I worked around by removing mozilla-openh264 before running the rebase again, at which point it succeeded:
rpm-ostree remove mozilla-openh264
#Fedora #fedora40 #Silverblue #upgrade #mozilla #OpenH264 #rpmOstree #error