With all respect to Lewanda's undoubted artistic skills, I'm rather afraid that your request would be beyond even her. I know the original artist uses sophisticated 3D modelling software to get the peeps' clothes, especially dresses, to 'flow' properly. Simply replacing the outfit with a dress wouldn't work. What would happen when the peep sits down, for instance?

It should be possible to change the colours of an existing outfit or dress, but not change it completely. I'd be very happy to be proved wrong, though!