They can be rerofit into an exhisting unit, it will however require removing the exsisting pull down screen. It pretty much goes in like a traditional door does with the outer case screwing into the walls, roof, and floor. The retail cost on the option is $1049.00 if ordered from the factory but shipping will be a lot of money because of the size and weight. Your best bet is to try to purchase it through a local dealer who has some toy haulers on order that we could unit load the 3 seasons wall into the back of a dealers stock unit if not the shipping cost will probobly be close to the cost of the actual wall.