If you still have originals without watermarks then regenerate the watermarks with different colors and styles of fonts.
If you don’t have any versions of the images other than the ones with watermarks, then those watermarks aren’t really watermarks anymore, they are just part of the images just like any other subject in the image, so you would deal with them the way you deal with any other unwanted image details, by replacing it with other image details via a clone or heal brush or hand-painting in details that you make up entirely from your mind.