Hello, I just bought the base game and immediately regret it because I didn't realize the hero edition had an actual campaign on top of the normal goodies.

Is there an option to upgrade my base game to hero edition so I can get the prequel campaign?

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.