The dress of Sydney did just that
To get this preppy casual look you might want to dress all your ladies in dresses from one of these preppy spots: Lilly Pulitzer , Sail to Sable (for a casual style!) All for Color , Jude Connally , Red Dress Boutique, J.Crew or Eliza J
Whether that means taking in a gown to make it smaller for your hips, bust, or waist (or maybe everywhere), adding cups, shortening straps, hemming the length, and adding in the bustle for your train, alterations are a standard part of the whole process
58" from HPS -Standard
Elizabeth Cole Crystaria Earrings Madewell Wide Cuff Bracelet BaubleBar Pearl & Faux Semi Precious Stone Wrap Necklace Don't: Wear a Floor-Length Gown Do: Wear a Mini or Midi Dress There is such a thing as going too formal
One detail worth remembering: tie width should roughly match your jacket lapel width