From what i understand from back then, SA asked Wayne N. to come back. the producers wanted him to come back, but each time he said no. so after the last overture, she started looking for the new RB. when she casted DH, she felt she found him...but if he didn;t have chemistry with Dee, he would have turned out to be one of Stefano's sons (which they were going to do when they were trapped on the island after Lawrence A. caused and earthquake.)
If Phil Capice had not been forced out as EP (and had Patrick Duffy not have returned) Steve Forrest's character Ben Stivers would have turned out to be Jock Ewing.