If you are set to reclass, do you have to pay back your bonus ?

Only if the conditions of your reclass involve you writing a statement saying you voluntarily submit to have your bonus recouped. Your normal, run of the mill reclass inside the reenlistment window is exempt from this.

If your bonus is tied to your current contract/MOS and the payments haven't finished paying out, then chances are you may have to pay back. Your Retention NCO can easily clear this up for you.

Depends on your reclass, some jobs yes pay it back, others no you keep it all. Talk to your Career Counselor.