If I already conquer a battlefield in my turn, retreat with the units in it, and then come back with another unit, does that unit get the "when I conquer" trigger?

No, your unit will not trigger its "When I conquer" ability.

According to Rule 447, a player may only score from a battlefield once per turn, regardless of the method (Conquer or Hold). Because "Conquer" is a subtype of scoring, you cannot perform a Conquer action on a battlefield that you have already scored from earlier in the same turn.

As established in the Riftbound rules and clarified by multiple FAQs (such as FAQ #1689 and FAQ #2782):

• Conquering is a subset of scoring: If you cannot score (because you have already scored that battlefield this turn), you cannot trigger a Conquer.
• Trigger Requirements: "When I conquer" abilities are triggered by the act of successfully Conquering a battlefield and gaining a point (or the equivalent scoring action). Since you are restricted to scoring each battlefield only once per turn, any subsequent movement into that battlefield—even if it results in you gaining control of it again—does not count as a Conquer and will not trigger your abilities.

In summary: Once you have scored a battlefield (via Hold or Conquer) during your turn, you cannot trigger "When I conquer" effects on that battlefield again for the remainder of that turn.