Updating deployment to increase or decrease canary size

In the current situation, an update to the job specification creates a new deployment, removing any existing canaries and deploying the new value.

A feature the devs have requested is that it is possible to increase the canary size of an existing deployment without triggering a new deployment.

The issue names no files, tests, or entry points to inspect. First clarify how an existing deployment should change its canary size without creating a new deployment, then identify the relevant deployment update path and tests that would demonstrate the behavior.

Written by the indexing model from the issue text.