For a 56 year old having sailed for fun with his daughter for just a year before the games, yes. I would definately call it a successful transition to cats. I would call anybody with a Kielerwoche win, an European championship and a fourth at the olympics, missing a bronze medal to Rob White in the last race, a very successful catamaran sailor. He did even better in other classes which was more homogenous, but that does not mean that he was not a succesful catamaran racer. You dont go to the olympics in any class without being an successful racer in the class, in my opinion (he beat the norwegian team, and I know what effort they made and how good those guys are).
Elvstrom could probably still beat both you and me speedwise on an F-16 (just dont make that long man do manouvers at his age).