Jaraxxus and Amara, Warden of Hope are both cards that change your hero's maximum life total. If you play one of the new Death Knight Heroes after having played Jaraxxus or Amara, what happens to your life total? Would your max health be returned to 30 or would you keep the 15/40 max life given by Jaraxxus/Amara?
DK heroes don't change your health value like Jaraxxus and Amara. They do, however, give you a bit of armor. If you're Jaraxxus at 13 HP and you play a DK card, you'll simply to go 13 HP and 4-6 armor (depending on the DK).