1) deply ceph-mon/radosgw/osd with from charm store and focal(source=distro)
bundle https://pastebin.canonical.com/p/Rx42Dg5Cvy/

2) switch from cs to ch 
juju refresh --switch ch:ceph-mon --channel octopus/stable ceph-mon
juju refresh --switch ch:ceph-radosgw --channel octopus/stable ceph-radosgw
juju refresh --switch ch:ceph-osd --channel octopus/stable ceph-osd

3) upgrade charm from octopus/stable to pacific/stable

juju refresh --channel pacific/stable ceph-mon
juju refresh --channel pacific/stable ceph-radosgw
juju refresh --channel pacific/stable ceph-osd

4) change UCA from distro to focal-wallaby

juju config ceph-mon source=cloud:focal-wallaby


juju run -a ceph-mon cat /etc/apt/sources.list.d/cloud-archive.list
cat: /etc/apt/sources.list.d/cloud-archive.list: No such file or directory


tail -f /var/log/juju/unit-ceph-mon-0.log 
2023-06-04 15:38:50 INFO unit.ceph-mon/0.juju-log server.go:316 running ceph osd set noout
2023-06-04 15:38:50 INFO unit.ceph-mon/0.juju-log server.go:316 DEPRECATION WARNING: Function action_set is being removed : moved to function_set()
2023-06-04 15:39:16 INFO unit.ceph-mon/0.juju-log server.go:316 Updating status.
2023-06-04 15:39:17 INFO juju.worker.uniter.operation runhook.go:159 ran "update-status" hook (via explicit, bespoke hook script)
2023-06-04 15:40:38 INFO unit.ceph-mon/0.juju-log server.go:316 old_version: octopus
2023-06-04 15:40:38 INFO unit.ceph-mon/0.juju-log server.go:316 new_version: None
2023-06-04 15:40:38 ERROR unit.ceph-mon/0.juju-log server.go:316 Invalid upgrade path from octopus to None.  Valid paths are: ['firefly -> hammer', 'hammer -> jewel', 'jewel -> luminous', 'luminous -> mimic', 'mimic -> nautilus', 'nautilus -> octopus']

juju status
Model  Controller  Cloud/Region       Version  SLA          Timestamp
ceph   mycontroller        stsstack/stsstack  2.9.42   unsupported  15:53:16Z

App           Version  Status  Scale  Charm         Channel         Rev  Exposed  Message
ceph-mon      15.2.17  active      1  ceph-mon      pacific/stable  152  no       Unit is ready and clustered
ceph-osd      15.2.17  active      2  ceph-osd      pacific/stable  548  no       Unit is ready (1 OSD)
ceph-radosgw  15.2.17  active      1  ceph-radosgw  pacific/stable  544  no       Unit is ready

Unit             Workload  Agent  Machine  Public address  Ports   Message
ceph-mon/0*      active    idle   0        10.5.0.145              Unit is ready and clustered
ceph-osd/0*      active    idle   1        10.5.1.212              Unit is ready (1 OSD)
ceph-osd/1       active    idle   2        10.5.1.204              Unit is ready (1 OSD)
ceph-radosgw/0*  active    idle   3        10.5.3.235      80/tcp  Unit is ready

Based on my last test [1], if I upgrade ceph-mon's charm from cs to quincy/stable directly, source=cloud:focal-wallaby can generate a correct cloud-archive.list file, cantaining Pacific UCA
But the charm upgrade path is not supported or recommended, as the official doc [2] mentions "The Ceph charms have been tested for single-step charm upgrades only (N+1)", cs:ceph-mon to quincy/stable is N+2(skip pacific/stable)



[1] https://bugs.launchpad.net/charm-ceph-mon/+bug/1951094/comments/10
[2] https://ubuntu.com/ceph/docs/upgrading-charms