Ray,
I still cannot reproduce this. Can you expand on "the old keys might not be valid"? AFAICT ceph charm doesn't remove keys upon broken client relation. In fact, ceph charm doesn't have a client-relation-departed hook. And next time, the client rejoins the relation with ceph, the charm client-relation-joined hook will try "ceph ... auth get-or-create <service>" key, which means if a key was created before for a service it will be simply returned again.

Below is the bundle I created for testing.

openstack-services:
  series: trusty
  services:
    mysql:
      branch: lp:charms/mysql
      constraints: mem=1G
      options:
        dataset-size: 50%
    rabbitmq-server:
      branch: lp:charms/rabbitmq-server
      constraints: mem=1G
    ceph:
      branch: lp:charms/ceph
      num_units: 2
      constraints: mem=1G
      options:
        monitor-count: 2
        fsid: 6547bd3e-1397-11e2-82e5-53567c8d32dc
        monitor-secret: AQCXrnZQwI7KGBAAiPofmKEXKxu5bUzoYLVkbQ==
        osd-devices: /dev/vdb
        osd-reformat: "yes"
        ephemeral-unmount: /mnt
    keystone:
      branch: lp:charms/keystone
      constraints: mem=1G
      options:
        admin-password: openstack
        admin-token: ubuntutesting
    glance:
      branch: lp:charms/glance
      constraints: mem=1G
  relations:
    - [ keystone, mysql ]
    - [ glance, mysql ]
    - [ glance, keystone ]
    - [ glance, ceph ]

After juju destroy-relation & add-relation, an image was created successfully. 

ubuntu@cbjchen-bastion:~/openstack-charm-testing$ juju destroy-relation ceph glance
ubuntu@cbjchen-bastion:~/openstack-charm-testing$ glance image-create --name cirros26 --disk-format qcow2 --container-format bare --is-public True --file ../images/cirros 
Request returned failure status.
500 Internal Server Error
Failed to upload image 35b3f5f4-8250-49f8-af39-f97dd2722091
    (HTTP 500)
ubuntu@cbjchen-bastion:~/openstack-charm-testing$ juju add-relation ceph glance
ubuntu@cbjchen-bastion:~/openstack-charm-testing$ glance image-create --name cirros26 --disk-format qcow2 --container-format bare --is-public True --file ../images/cirros 
+------------------+--------------------------------------+
| Property         | Value                                |
+------------------+--------------------------------------+
| checksum         | 133eae9fb1c98f45894a4e60d8736619     |
| container_format | bare                                 |
| created_at       | 2014-11-18T21:50:27                  |
| deleted          | False                                |
| deleted_at       | None                                 |
| disk_format      | qcow2                                |
| id               | d008db19-8281-4394-9a58-47e179d7f858 |
| is_public        | True                                 |
| min_disk         | 0                                    |
| min_ram          | 0                                    |
| name             | cirros26                             |
| owner            | 092e164fbef64890ae933a988c57d023     |
| protected        | False                                |
| size             | 13200896                             |
| status           | active                               |
| updated_at       | 2014-11-18T21:50:28                  |
| virtual_size     | None                                 |
+------------------+--------------------------------------+


Please let me know if there are any steps that I missed in order to manifest this issue? Thanks.