If you are a user of Prometheus 1.x, the following information may be relevant.

Prometheus 2 contains a number of breaking changes, including:
  - Changes to how command line flags are parsed (double dash is now required)
  - Removal of some command line flags
  - Alert Manager options moved into prometheus.yaml
  - Alerting / Recording Rules format changes
    - Rules are now in YAML format
  - Changes to the storage engine
    - These changes mean that old data will be unreadable by Prometheus 2,
      refer to the Prometheus 2 migration guide for more information on this
      issue

Any existing Prometheus 1 `prometheus.yml` should continue to work with
Prometheus 2.

As a safety measure, the Prometheus 2 init script will refuse to start
Prometheus if it finds a Prometheus 1 data dir at the $prometheus_data_dir
location.

You will likely wish to review the Prometheus 2 migration guide at
https://prometheus.io/docs/prometheus/2.1/migration/

Brief migration steps:
  - Stop the running Prometheus 1.
  - Move any existing `prometheus_` rcconf variables to their equivalent
   `prometheus1_` rcconf variable.
  - Move the Prometheus 1 data dir to a suitable location.
    - A new $prometheus_data_dir for Prometheus 2 will be created by the init
      script.
  - Set the `prometheus1_data_dir` rcconf variable to the location that you
    moved the Prometheus 1 data dir to.
  - Copy your existing `prometheus.yml` to `prometheus1.yml`.
    - Prometheus 2 should be able to use your existing Prometheus 1 config.
  - Put Prometheus 1 into a read-only mode on a different port making changes
    described at: https://prometheus.io/docs/prometheus/2.0/migration/#storage
    - You can use the `prometheus1_args` rcconf setting to pass additional
      arguments.
  - Add a `remote_read` section to the `prometheus.yml` used by Prometheus 2,
    this allows Prometheus 2 to have access to the metrics previously gathered
    by Prometheus 1.
  - Start both Prometheus 1 and 2.