To originally cast the spell Hex you must choose a target you can see within range. However, the spell says that a player can use a bonus action on a subsequent turn to move the curse to a new creature. However, it does not say that the new creature being cursed has to be seen or within range. Rules as Written that seems correct, but isn't that a bit overpowered?
The "see within range" part still applies.
The spell states:
You place a curse on a creature that you can see within range. [description of effects]
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 curse a new creature.
It allows you to apply the curse to another creature. The procedure of applying this curse is described earlier in the spell's description, and the phrasing limits the target selection to be within range and sight.
Cursing a target when you cast the spell is no different from moving it to another creature later on, at least not concerning target validity.
I suppose that RAW, you could read this differently if you were intent on it. However, common sense tells us that the spell clearly isn't supposed to work the way you asked.