Shortcode to Reference Pre-Alteration Data

Problem: When a reservation field changes (e.g., guest count, check-in/out date), notifications can only show the new value. We need to also show the previous value to make changes obvious and auditable.

Request: Add shortcodes that expose both previous and current values for any tracked field in notification rules and message templates.

Example: Alteration of guest counts from number of guests: 1 to number of guests: 2. Because currently the alteration notification shows the number of guests: 2