Watchface Scheduler makes your Pebble watchfaces change automatically, allowing you to make the most out of thousands of great designs.

The app allows you to manually enter watchfaces with their UUID, or import them from Pebble support logs.
You can then schedule a selection of your watches to automatically rotate at a specified interval, or schedule changes for a specific time & day.

Here's a review of the app:
http://www.pebblestuff.io/apps/watchface-scheduler-automatically-switch-pebble-watchfaces/

To import your watchfaces:
1. Open up the Pebble app and click on support from the menu
2. Click "Contact Support", select any subject then select "Import Pebble App UUIDs" from the list when prompted
OR
Send the support email to yourself and open the "pebble.log.gz" attachment with this application.

WARNING: The Pebble Time app is required for watchface import. If you have an original Pebble or Pebble Steel, you will need to install the Pebble Time app and update to firmware 3.8 to use this feature.