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?