As far as I know neither GOG or Steam let you upgrade the standard edition to the Hero edition: I suppose it's a publisher's (controversial) decision.

Since you have bought the game recently, I think you could try asking for a refund even if you don't meet the requirements (read here), in order to buy the Hero edition instead. I think GOG would be sympathetic, if you explain them your situation.
On the other hand, even if you immediately write to GOG support, they will probably not reply to your request before present sale's end: so you could lose the current discount.