If you take an upgrade you'll pay the prorated amount of the new annual fee.

For example, if you pay your Green card $150 annual fee in January and upgrade to the Gold card in July you would pay ($250 - $150) / 2 = $50. A little more complicated as AMEX prorates the fee by day, not month, but that's the gist.

What you should consider doing though to get the *most* points, assuming you've never had a Gold card before, is apply for a new Gold card with the 75k SUB and then quickly upgrade your green card to get the upgrade bonus as well. Yes you'll have 2 Gold cards, but you'll get 115k bonus points. If you take the upgrade offer you won't be eligible for the 75k bonus until 7 years after you cancel your Gold card.