Comment 18 for bug 1769669

@Renat: You can use a local .snap file with --extra-snaps for a snap named in the model (for example core, or pi2-kernel, or your gadget) and the snap at runtime is refreshable (that is it can be updated). I just tried it with an older core snap like this:

ubuntu-image -O img-2 --extra-snaps core_4489.snap model

At runtime I got this:
knitzsche@localhost:~$ snap info core
name: core
summary: snapd runtime environment
publisher: canonical
license: unknown
description: |
  The core runtime environment for snapd
type: core
snap-id: 99T7MUlRhtI3U0QFgl5mXXESAiSwt776
tracking: stable
refreshed: 2018-04-16T10:40:40Z
installed: 16-2.32.5 (4489) 76MB core
channels:
  stable: 16-2.32.6 (4573) 76MB -
  candidate: 16-2.32.6 (4573) 76MB -
  beta: 16-2.32.6 (4573) 76MB -
  edge: 16-2.32.6+git711.8d0e194 (4628) 76MB -

And I ran 'snap refresh core' and it is refreshing to 4573.