I did much the same first time out but in the opposite direction, misread the idea of replacing the brewing sugar with malt extract and ended up doubling it
Increasing the malt or sugar at the secondary fermentation won’t give you too much more abv, but will risk over fermenting your bottles or keg and could result in a nasty mess and wasted effort because of the over pressurisation. I’d take the reduced abv and chalk it up to experience. What ever you do enjoy the result, I’m sure it’ll be drinkable