Can you conquer a Battlefield where you scored a Hold already and moved your unit back this turn?

No, you cannot. According to Rule 465 and multiple RiftJudge FAQs (such as FAQ #3369 and #2805), a player may only score a given battlefield once per turn, regardless of the method used (Hold or Conquer).

Because you already scored the battlefield via a Hold at the beginning of your turn, you have reached the scoring limit for that location for the remainder of the turn. Moving your unit away and back to that same battlefield does not reset this restriction, and you cannot conquer it to score again or trigger conquer effects.