Despite what some sources say, previews started on Oct. 8, 1981, not Oct. 1, 1981. For confirmation, see this contemporaneous article from the New York Times. The opening was originally scheduled for November 1. This was first postponed to November 9, then to November 16.
Jim Walton replaced James Weissenbach in the leading role of Franklin Shepard on Oct. 19. At that time, Walton was replaced in his original role of Jerome by David Cady, who was the understudy for Franklin Shepard all along. Walton was never listed as the understudy for Franklin Shepard.
The character of older Frank was added to the show late in previews. If a contemporaneous article in the New York Times is to be trusted, the first performance with the character was on Tuesday, Nov. 3. (The article reads a bit ambiguously and could be seen as meaning that the character was added to the show on Nov. 10, but the character was definitely in the show by Nov. 7.)