So in the old rules, Hunter's Mark said, if a creature drops to 0, you can move it as a bonus action "on a subsequent turn." The 2024 rules removed this "restriction", presumably meaning that you can move it on the same turn that an enemy drops. I've always taken this to be that if an ally causes an enemy to drop, then when your turn comes around again, you can move it to a new target. 

However, based on the new rules text, that doesn't include the subsequent turn clause, my DM is telling our Ranger that, if something drops, and it comes back around to his turn, he has to use another of his limited free uses or a spell slot to recast it. So it could only be moved if the ranger himself drops a unit. This feels really restrictive to a class that people regularly say is already limited/underpowered. Is anyone aware of a clarification that explains who's interpretation is intended? I know "the DM gets final say", but I want to help out my friend's argument if possible.

The text of Hunter's Mark reads on moving the spell:

If the target drops to 0 Hit Points before this spell ends, you can take a Bonus Action to move the mark to a new creature you can see within range.

And that's all it has to say. It doesn't matter when, why, or how the target dropped  to 0 hit points. If the Hunter's Mark target drops to 0, the Ranger has a Bonus Action available for use, and the spell is still active, he can use that Bonus Action to move the spell to a new creature within range.

By a strict reading of the rules, it doesn't even matter if the creature goes back to positive hit points before the Ranger decides to use his Bonus Action to move the spell.