Hate to be "that guy," but it comes down to what you mean by benevolent. Kindly and charitable leaves a lot of ambiguity. I think it's fair to say that the United States believes itself to be benevolent, but our history disagrees with that. Even the Federation has its dark side (what with every other Admiral being outright corrupt, Section 31, etc.).
If that's the case, I don't think the Jem'Hadar and Vorta would need to change that much. I can see them working to better the Gamma Quadrant, but still utilizing the Jem'Hadar when "bettering" means using force. Some situations, sadly, will not resolve themselves through good wishes and diplomacy. However, in the attempt, the Vorta would be sent in first to try to negotiate/seek a peaceful solution.
If you're talking "attempting to become part of the Federation," then both the Jem'Hadar and Vorta pose a significant issue both with the issues of slavery and the ban on genetic manipulation. So, the Founders wouldn't be able to further manipulate them to make them docile or even to give them autonomy. In that case, the Jem'Hadar would have to die out, as they had no way to procreate, and the Vorta, well, give them a planet of their own, I guess? Can they procreate? I don't know that the show gave us any answer to that. Furthermore, and argument could be made that the most benevolent and perhaps "humane" ("Even the very word is racist...") thing would be to euthanize the Jem'Hadar then to ween them off the white, unless the Founders had the solution Bashir desperately sought.
Then again, the most benevolent thing the Founders could do would be to release the many members, but as someone else pointed out, that could result in severe repercussions.
I guess I keep circling around the idea that a change to a benevolent Dominion would be a very messy affair. It would either have to be quick, with a lot of death and destruction or slowly weening the members away, but still maintaining control until they could survive on their own. That could take decades or even centuries.
Going back to my original point, maybe not the Founders, but I think it's clear the Vorta believed the Dominion was benevolent. They truly saw being a member of the Dominion as a net positive for the members, even if they did have to use an iron fist in a velvet glove to make that the case.