I think you would need the British to really drop the ball in the southern colonies so much that the Patriots could wrest them away from the Loyalists. With the union stretching from Montreal to Georgia instead of the southern boundary being the border of Virginia, slaveholding interests will have more power; yes, slavery existed in every state but Massachusetts IIRC in 1789, but it was only prominent south of Pennsylvania and most of the Virginian founding fathers were idealists and not entirely comfortable with slavery, and even then there was a lot of animosity from the planters because Jefferson and Washington prevented the state's delegates from walking out.