If I trigger a non-combat showdown on an empty battlefield, ride the wind my unit to base, and then my opponent uses focus to ride the wind their unit to the battlefield, at which point does it change from a non-combat to combat showdown?

Based on the rules of Riftbound and the established FAQs regarding showdowns, here is the ruling for your scenario:

Ruling

The showdown does not change from a non-combat to a combat showdown. A showdown cannot change its type once it has begun. Instead, the current non-combat showdown will resolve normally, and if both players have units present at the battlefield when it ends, a new combat showdown will be staged and begin immediately afterward.

Sequence of Events

1. Initial State: You trigger a non-combat showdown on an empty battlefield.
2. Your Action: You play Ride the Wind to move your unit to your base.
3. Opponent's Action: Your opponent plays Ride the Wind to move their unit to the battlefield.
4. Showdown Resolution: Players continue to pass focus in order until the current non-combat showdown ends.
5. Cleanup/Staging: Because both players now have units present at the battlefield, control cannot be established. A new Combat Showdown is staged and begins immediately.
6. Designations: In this new combat showdown:
   • The player who originally applied the "Contested" status to the battlefield becomes the Attacker.
   • The player who moved their unit to the battlefield second becomes the Defender.

Key Nuances

• Showdowns cannot change type: As stated in the FAQ, "Showdowns cannot change type mid-resolution."
• Staged Combat: When a non-combat showdown ends with units from two opposing players present, the game automatically transitions to a combat showdown.
• Attacker/Defender: The attacker is always the player who first applied the "Contested" status to that battlefield, regardless of the movement sequence.

According to Rule 345.2.b: "If two players’ Units remain at the Battlefield, Control cannot be established and the Battlefield remains Contested. This results in a Staged Combat."