Free is an edition, not a countdown
When a Free edition is available, you can install it without purchasing Pro. It has no expiry. The face page links only to editions approved in the latest export.
Pro is a separate store listing
Choose Pro on the face page to see its current local price on Garmin Connect IQ. It is a one-time purchase. Extra features are specific to each design; the Hyperdrive comparison, for example, covers its cockpit palettes, destinations and instruments.
Select the Pro face after installing
After purchasing and syncing Pro, choose that edition in your watch-face selection. You can remove the separate Free edition without removing Pro.
Check compatibility before changing editions
The two editions can have different approved device lists. Check the Free and Pro indicators beside your exact model on the product page.