The 255 will be slightly smaller tire in terms of diameter and circumference. Will affect the speedometer and odometer slightly.You can check the tire spec known as "Revolutions per mile" to get an exact comparison. Probably not a big deal.
Aslo check the dual spacing and make sure the slightly wider wider 255/80 will fit ok in the front wheel wells when the steering wheel is cramped fully left and right. The 255 is about 0.8" wider.
You can get all the dimensions from the spec entry for the tire in the manufacturers brochure or website. If it's a Goodyear or Michelin, it's also in their RV Tire Guide.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL