You are deploying a new release of your application with a schema change and are concerned about one-off dynos already running using the previous schema version.
One-off dynos, those created by
heroku run, executed by Heroku Scheduler or other means of starting "extra" dynos, are not killed when a new release is made; Only formation dynos,
worker, etc., those specified in your
Procfile and are able to be scaled, are restarted after a release finishes.
See https://devcenter.heroku.com/articles/dynos#dyno-configurations for additional information on dyno types.