When fighting multiple hunters as Dracula do you have to fight the same target the entire fight or can you switch targets during the fight?
You can choose a new target every round. Choosing a target is step 2 of the combat sequence occurring after cards are played in the round.
See the reference guide under combat
Combat is played over a series of rounds. Each round occurs in this order:
1. Choose Combat Cards...
2. Choose Engaged Hunter...
3. Reveal Combat Cards...