I can reproduce the agent being upgraded quite trivially
⟫ juju version
1.22.1-vivid-amd64
⟫ juju bootstrap --to my.machine
...
⟫ juju status --format=tabular
[Machines]
ID STATE VERSION DNS INS-ID SERIES HARDWARE
0 started 1.22.3 my.machine /MAAS/api/1.0/nodes/node-36b4d226-4b4a-11e4-a091-a0b3cce4ecca/ trusty arch=amd64 cpu-cores=4 mem=16384M
I can reproduce the agent being upgraded quite trivially
⟫ juju version 1.0/nodes/ node-36b4d226- 4b4a-11e4- a091-a0b3cce4ec ca/ trusty arch=amd64 cpu-cores=4 mem=16384M
1.22.1-vivid-amd64
⟫ juju bootstrap --to my.machine
...
⟫ juju status --format=tabular
[Machines]
ID STATE VERSION DNS INS-ID SERIES HARDWARE
0 started 1.22.3 my.machine /MAAS/api/