I have a unit on Star spring battlefield, enemy tries to conquer it. I play an ambush unit on it. Can I move my other unit there then?

Ruling

Yes, you can move your other unit to its base.

When you play an Ambush unit to a battlefield where you already have a unit, you trigger the ability on Star Spring. Because the Ambush unit is a non-token unit, playing it satisfies the condition for Star Spring's ability.

Sequence of Events

1. Enemy Action: Your opponent initiates a move to your battlefield to conquer it. This opens a Showdown.
2. Your Reaction: You play your Ambush unit to that battlefield. Because you already control a unit there, the battlefield is a valid location for the Ambush unit (Rule 822.1.b).
3. Triggered Ability: Playing the Ambush unit (a non-token unit) triggers Star Spring's ability: "The first time a player plays a non-token unit here each turn, they may move another unit they control here to its base."
4. Chain Resolution: This triggered ability is placed on the chain. You may choose to resolve it, allowing you to move your original unit back to your base.

Important Considerations

• Control: You do not lose control of the battlefield by moving your original unit to your base because the Ambush unit is still present there. As long as you have at least one unit at the battlefield, you maintain control (FAQ #9030, FAQ #6313).
• Ambush Timing: Ambush units are played as a [Reaction], which allows them to be played even while the game state is Closed (such as during a Showdown).
• Chain Interaction: The ability on Star Spring uses the chain. Once the Ambush unit enters play, the ability is added to the chain, and you have the opportunity to resolve it before the Showdown proceeds.

Note: I could not fully verify this ruling against the retrieved rules.