Does retreating all units from a battlefield you already scored this turn, then moving a unit back onto it, count as a 'Conquer' and trigger Conquer effects?

Ruling: No, it does not count as a Conquer. A Conquer only occurs when a player gains control of a battlefield they have not yet scored that turn.

Nuances:

• If an effect like Tianna prevents you from scoring for holding a battlefield, and you later remove that effect and gain control, you can still get a Conquer point since you haven't scored that battlefield yet this turn
• Holding scores only happen during the Beginning phase, so gaining control later in the turn won't trigger holding score