I am not quite sure how I ended up in a similar situation, but I have a Yoga cloud that was upgraded from Focal to Jammy. Today, while adding a new nova-compute unit I encountered this bug.

Upgrading my controller and models to 3.3.5 did not fix the issue, so I had to do a bit of database fix.

Documenting the process here in case it is useful for someone else:

$ juju set-application-base nova-compute ubuntu@22.04
ERROR updating application base: base "ubuntu@22.04" not supported by charm, the charm supported bases are: ubuntu@20.04, ubuntu@18.04

$ juju add-unit nova-compute --to 120 
ERROR acquiring machine to host unit "nova-compute/19": cannot assign unit "nova-compute/19" to machine 120: base does not match: unit has "ubuntu@20.04", machine has "ubuntu@22.04"

juju:PRIMARY> db.applications.find({"name":"nova-compute"});
{ "_id" : "98f30d5a-977c-4ce9-80ef-c97152b096f0:nova-compute", "name" : "nova-compute", "model-uuid" : "98f30d5a-977c-4ce9-80ef-c97152b096f0", "subordinate" : false, "charmurl" : "ch:amd64/focal/nova-compute-740", "charm-origin" : { "source" : "charm-hub", "type" : "charm", "id" : "ubPbvtErosR9P4NGEt24wak8LDsczRi4", "hash" : "cc089c4492344dc6301b40a1b8ec7c7fd3280f273053b31c3b18e945c87e2f15", "revision" : 740, "channel" : { "track" : "yoga", "risk" : "stable" }, "platform" : { "architecture" : "amd64", "os" : "ubuntu", "channel" : "20.04" } }, "charmmodifiedversion" : 11, "forcecharm" : false, "life" : 0, "unitcount" : 16, "relationcount" : 16, "minunits" : 0, "txn-revno" : NumberLong(19), "metric-credentials" : BinData(0,""), "exposed" : false, "scale" : 0, "passwordhash" : "", "provisioning-state" : null }

juju:PRIMARY> db.units.find({"name":"nova-compute/19"});
{ "_id" : "98f30d5a-977c-4ce9-80ef-c97152b096f0:nova-compute/19", "name" : "nova-compute/19", "model-uuid" : "98f30d5a-977c-4ce9-80ef-c97152b096f0", "base" : { "os" : "ubuntu", "channel" : "20.04/stable" }, "application" : "nova-compute", "charmurl" : null, "principal" : "", "subordinates" : [ ], "storageattachmentcount" : 0, "machineid" : "", "resolved" : "", "life" : 0, "passwordhash" : "", "txn-revno" : 2 }

(Find All)
juju:PRIMARY> db.units.find({ "name": { $regex: /^nova-compute\/.*/} });


(Override Focal > Jammy, keeping the charmurl intact)

juju:PRIMARY> db.applications.updateOne({"name":"nova-compute"}, { $set: { "name" : "nova-compute", "model-uuid" : "98f30d5a-977c-4ce9-80ef-c97152b096f0", "subordinate" : false, "charmurl" : "ch:amd64/focal/nova-compute-740", "charm-origin" : { "source" : "charm-hub", "type" : "charm", "id" : "ubPbvtErosR9P4NGEt24wak8LDsczRi4", "hash" : "cc089c4492344dc6301b40a1b8ec7c7fd3280f273053b31c3b18e945c87e2f15", "revision" : 740, "channel" : { "track" : "yoga", "risk" : "stable" }, "platform" : { "architecture" : "amd64", "os" : "ubuntu", "channel" : "22.04" } }, "charmmodifiedversion" : 11, "forcecharm" : false, "life" : 0, "unitcount" : 16, "relationcount" : 16, "minunits" : 0, "txn-revno" : NumberLong(19), "metric-credentials" : BinData(0,""), "exposed" : false, "scale" : 0, "passwordhash" : "", "provisioning-state" : null } } );
{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

juju:PRIMARY> db.units.updateMany({ "name": { $regex: /^nova-compute\/.*/ } }, { $set: { "base" : { "os" : "ubuntu", "channel" : "22.04/stable" } } });
{ "acknowledged" : true, "matchedCount" : 16, "modifiedCount" : 16 }

$ juju add-unit nova-compute --to 120

$ juju status nova-compute/20
Model   Controller           Cloud/Region      Version  SLA          Timestamp
vstack  home-lab-controller  Home-Lab/default  3.3.5    unsupported  13:48:19-07:00

SAAS        Status  Store                 URL
prometheus  active  openstack-controller  admin/grafana.prometheus2

App                        Version  Status       Scale  Charm                Channel        Rev  Exposed  Message
canonical-livepatch                 blocked          1  canonical-livepatch  latest/stable   49  no       Activation failed
lds-client-jammy                    waiting        0/1  landscape-client     latest/stable   69  no       agent initialising
masakari-monitors                   waiting        0/1  masakari-monitors    yoga/stable     54  no       agent initialising
masakari-pacemaker-remote           maintenance      1  pacemaker-remote     jammy/stable    23  no       installing charm software
nova-compute               25.2.1   blocked          1  nova-compute         yoga/stable    740  no       Missing relations: vault, image, messaging
ovn-chassis                         waiting        0/1  ovn-chassis          22.03/stable   290  no       agent initialising
telegraf                            waiting        0/1  telegraf             latest/stable   75  no       agent initialising

Unit                             Workload     Agent       Machine  Public address  Ports  Message
nova-compute/20                  blocked      executing   120      10.1.8.36              Missing relations: vault, image, messaging
  canonical-livepatch/34         blocked      executing            10.1.8.36              (install) Activation failed
  lds-client-jammy/695           waiting      allocating           10.1.8.36              agent initialising
  masakari-monitors/123          waiting      allocating           10.1.8.36              agent initialising
  masakari-pacemaker-remote/126  maintenance  executing            10.1.8.36              (install) installing charm software
  ovn-chassis/79                 waiting      allocating           10.1.8.36              agent initialising
  telegraf/237                   waiting      allocating           10.1.8.36              agent initialising

Machine  State    Address    Inst id   Base          AZ       Message
120      started  10.1.8.36  os-vm-16  ubuntu@22.04  default  Deployed