In the description for the Hunter's Mark spell, it says:
If the target drops to 0 hit points before this spell ends, you can
use a bonus action on a subsequent turn of yours to mark a new
creature.
That seems clear, and it's how we've been running it: the target's HP must be brought down to 0 before the mark can be moved. We haven't found anything in RAW specifically saying you can't move it, but it doesn't seem to fit the spirit/intent of the spell.
But tonight, something unexpected happened and we were not 100% sure how to rule it. A target marked by the party's Ranger decided it had taken enough arrows and Plane Shifted elsewhere. While the ranger's Hunter's Mark spell was still active, there was no way to track the target at this point, either. Does she have to recast it to mark a new creature, or is she able to move the mark to another target, since the original target is, for all intents and purposes, gone?
Best Answer
You would have to recast the hunter's mark spell
The hunter's mark spell states:
The spell describes no other times you can change the target besides when it drops to 0 hit points. There are also no conditions described where the spell outright ends besides itss duration running up.
The only way to change the target of the spell besides the usual method is to cast it again, and because hunter's mark is a concentration spell, the first casting would end.