@dp20eic auch moin :-), danke für Deine Rückmeldung.
Ja den check hatte ich natürlich gemacht, ohne Auffälligkeiten, gerade aktuell nochmal und dabei kommt das heraus:
(meine Syno (=Backup) schläft um die Zeit noch und die anderen Warnings kommen ja von den Containern, welche ich vor dem Upgrade-Versuch aber alle manuell gestoppt hatte):
root@proxo:~# pve7to8
= CHECKING VERSION INFORMATION FOR PVE PACKAGES =
Checking for package updates..
PASS: all packages up-to-date
Checking proxmox-ve package version..
PASS: proxmox-ve package has version >= 7.4-1
Checking running kernel version..
PASS: running kernel '5.15.116-1-pve' is considered suitable for upgrade.
= CHECKING CLUSTER HEALTH/SETTINGS =
SKIP: standalone node.
= CHECKING HYPER-CONVERGED CEPH STATUS =
SKIP: no hyper-converged ceph setup detected!
= CHECKING CONFIGURED STORAGES =
storage 'BackUp-Proxo' is not online
WARN: storage 'BackUp-Proxo' enabled but not active!
PASS: storage 'datatransfer' enabled and active.
PASS: storage 'local' enabled and active.
PASS: storage 'local-lvm' enabled and active.
INFO: Checking storage content type configuration..
PASS: no storage content problems found
WARN: activating 'BackUp-Proxo' failed - storage 'BackUp-Proxo' is not online
PASS: no storage re-uses a directory for multiple content types.
= MISCELLANEOUS CHECKS =
INFO: Checking common daemon services..
PASS: systemd unit 'pveproxy.service' is in state 'active'
PASS: systemd unit 'pvedaemon.service' is in state 'active'
PASS: systemd unit 'pvescheduler.service' is in state 'active'
PASS: systemd unit 'pvestatd.service' is in state 'active'
INFO: Checking for supported & active NTP service..
PASS: Detected active time synchronisation unit 'chrony.service'
INFO: Checking for running guests..
WARN: 5 running guest(s) detected - consider migrating or stopping them.
INFO: Checking if the local node's hostname 'proxo' is resolvable..
INFO: Checking if resolved IP is configured on local node..
PASS: Resolved node IP '192.168.188.100' configured and active on single interface.
INFO: Check node certificate's RSA key size
PASS: Certificate 'pve-root-ca.pem' passed Debian Busters (and newer) security level for TLS connections (4096 >= 2048)
PASS: Certificate 'pve-ssl.pem' passed Debian Busters (and newer) security level for TLS connections (2048 >= 2048)
INFO: Checking backup retention settings..
PASS: no backup retention problems found.
INFO: checking CIFS credential location..
PASS: no CIFS credentials at outdated location found.
INFO: Checking permission system changes..
INFO: Checking custom role IDs for clashes with new 'PVE' namespace..
PASS: no custom roles defined, so no clash with 'PVE' role ID namespace enforced in Proxmox VE 8
INFO: Checking if LXCFS is running with FUSE3 library, if already upgraded..
SKIP: not yet upgraded, no need to check the FUSE library version LXCFS uses
INFO: Checking node and guest description/note length..
PASS: All node config descriptions fit in the new limit of 64 KiB
PASS: All guest config descriptions fit in the new limit of 8 KiB
INFO: Checking container configs for deprecated lxc.cgroup entries
PASS: No legacy 'lxc.cgroup' keys found.
INFO: Checking if the suite for the Debian security repository is correct..
PASS: found no suite mismatch
INFO: Checking for existence of NVIDIA vGPU Manager..
PASS: No NVIDIA vGPU Service found.
INFO: Checking bootloader configuration...
SKIP: not yet upgraded, no need to check the presence of systemd-boot
SKIP: NOTE: Expensive checks, like CT cgroupv2 compat, not performed without '--full' parameter
= SUMMARY =
TOTAL: 32
PASSED: 24
SKIPPED: 5
WARNINGS: 3
FAILURES: 0
ATTENTION: Please check the output for detailed information!
Installiert hatte ich das ganze System vor einiger Zeit komplett neu, also nicht auf einem bestehendem Image.
Leider habe ich aber keinen Link mehr dahin, wobei es aber ein offizielles Proxmox Image war und ich danach dann die Quellen nach einem Tutorial auf 'nicht Enterprise' umgehängt hatte. Zu dem Zeitpunkt kannte ich leider noch nicht die Anleitung aus der ioBroker Doku.
Den Hinweis bzgl.
Upgrade wants to remove package 'proxmox-ve
hatte ich auch bereits gefunden und auch schon befolgt:
root@proxo:~# sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list
root@proxo:~# apt remove linux-image-amd64
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Note, selecting 'pve-kernel-5.15' instead of 'linux-image-amd64'
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Danach kommt zwar keine Fehlermeldung mehr, aber es läuft auch kein Update:
root@proxo:~# apt dist-upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
root@proxo:~# apt full-upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Wobei wenn ich jetzt oben lese:
pve-kernel-5.15
Bin ich da etwa "unter der Motorhaube" noch auf irgendeiner Alt-Version?
In der Kopfzeile meines Proxmox steht auf jeden Fall:

Edit: Jetzt hab ich's wieder. Von hier hatte ich das Image gezogen:
https://www.proxmox.com/de/proxmox-virtual-environment/erste-schritte
... einen bootfähigen USB-Stick erstellt und dann den HP Elitedesk davon gebootet und installiert.