I upgraded my account from standard edition to heroic edition. According to the advertisement in the little yellow banner, a level 70 character boost is included with that, and I have yet to receive the character boost, although I have received everything else that came with the heroic edition upgrade, at least to my knowledge.
Semper Fi! 
You would have already received the boost with the initial purchase. An upgrade will not provide another one.
Upgrading to the Heroic or Epic Edition will not re-grant in-game items previously unlocked on your account.
Shop link: https://us.shop.battle.net/en-us/product/world-of-warcraft-the-war-within
Upgrading the Base or Heroic edition to Epic of a specific expansion does not grant an additional boost. If you want an additional boost after already owning one from an expansion pack, a separate Character Boost service is available for purchase.
Yet the little yellow banner says that a level 70 character boost will be included with all editions. So yes, as per the advertisement, I should have received a level 70 character boost with the heroic edition upgrade. Truth in advertisement. I’m pretty sure it is illegal to advertise something being included with a purchase and then not provide it. If not, then it should be.
Semper Fi!
It is received with all editions. You already received it when you bought the earlier edition.
All stand-alone editions include a boost.
However, as stated on the Shop page that I linked earlier, upgrading to a higher edition will not re-grant previously awarded items.