An original cast member (and one of the reasons Larry Hagman signed in to play J.R. Ewing), Barbara Bel Geddes portrayed family matriarch Miss Ellie for six seasons.

Following an emergency quadruple bypass, she needed to take a leave of absence, and Donna Reed stepped in for a temporary spell.

https://tvline.com/lists/recast-tv-roles-explained/family-matters-harriette-winslow/