Bargnani will be a post up threat against undersized fours.
Other than that, his shot-creation abilities will take a hit by switching to the four ... but that doesn't matter a whole pile because he is a weak shot-creator to begin with.
A.Bargnani relies heavily on his teammates to create quality shot attempts for him. He needs teammates who can attract defensive attention and find him for open looks. Dwight Howard will be very good at that + they should be able to add some dribble penetration to further help Bargnani.
Defensively, Bargnani will be a victim against anyone with average to above average quickness at his position. Against slow PFs or PFs with limited offensive range, he'll be an effective individual defender. He'll also be a consistently abysmal help defender + his inability to defend his position adequately will lead to an increased number of situations where Dwight Howard can find himself in foul trouble.
Plus, his rebounding will decline. Further from the bucket. Doesn't explode to the ball. Incapable of boxing out most PFs because he doesn't have enough quickness, agility or commitment to do so.