Send email when entity field value is changed (Drupal)

You can use Rules to achieve this.

1. Create new rule that will react on “After updating existing content” event.
2. Use the following conditions: data comparison: node:field-your-field and node-unchanged:field-your-field and select “Negate: FALSE”
3. Create whichever actions you want, e.g. send mail, show message, etc.