Some variant of the following[1] worked for me.

The first line is the start of a for loop that runs on each node in my cluster a command using ssh. The argument -t is passed to attach a controlling terminal to STDIN, STDERR and STDOUT of this session, since there will not be an intervening shell to do it for us. The argument to ssh is a workflow of bash commands. They upgrade the 7.x system to the most recent packages on the repository. We then update the sources.list entries for the system to point at bookworm sources instead of bullseye. The package cache is updated and the proxmox-ve package is installed. Packages which are installed are upgraded to the versions from bookworm, and the installer concludes.

Dear reader, you might be surprised how many times I saw the word “perl” scroll by during the manual, serial scrolling of this install. It took hours. There were a few prompts, so stand by the keyboard!

[1]

gpg: key 1140AF8F639E0C39: public key "Proxmox Bookworm Release Key " imported
# have your ssh agent keychain running and a key loaded that's installed at 
# ~root/.ssh/authorized_keys on each node 
apt-get install -y keychain
eval $(keychain --eval)
ssh-add ~/.ssh/id_rsa
# Replace the IP address prefix (100.64.79.) and  suffixes (64, 121-128)
# with the actual IPs of your cluster nodes.  Or use hostnames :-)
for o in 64 121 122 123 124 125 126 127 128 ; do   ssh -t root@100.64.79.$o '
  sed -i -e s/bullseye/bookworm/g /etc/apt/sources.list $(compgen -G "/etc/apt/sources.listd.d/*.list") \
  && echo "deb [signed-by=/usr/share/keyrings/proxmox-release.gpg] http://download.proxmox.com/debian/pve bookworm pve-no-subscription" \
    | dd of=/etc/apt/sources.list.d/proxmox-release.list status=none \
  && echo "deb [signed-by=/usr/share/keyrings/proxmox-release.gpg] http://download.proxmox.com/debian/ceph-quincy bookworm main no-subscription" \
    | dd of=/etc/apt/sources.list.d/ceph.list status=none \
  && proxmox_keyid="0xf4e136c67cdce41ae6de6fc81140af8f639e0c39" \
  && curl "https://keyserver.ubuntu.com/pks/lookup?op=get&search=${proxmox_keyid}" \
    | gpg --dearmor -o /usr/share/keyrings/proxmox-release.gpg  \
  && apt-get -y -qq update \
  && apt-get -y -qq install proxmox-ve \
  && apt-get -y -qq full-upgrade \
  && echo "$(hostname) upgraded"'; done