Sometimes the understudy is too old or too tall to take over and sometimes they just like to find someone totally new, who can stay with the show longer.
Randall Anne Brooks was Sarah Jessica Parker's understudy. Randall Anne Brooks came on this forum many years ago and explained what happened. I think I remember she said she was informed many months before her last show that she was too tall replace Sarah. I think Randall Anne was like 5'2" when her contract ended.
January 1980 - It was a new decade, and Martin decided the show needed some shaking up and revitalization, so he fired almost the whole cast all at once, including Robert Fitch, who had originated the role of Rooster. These cast members were NOT happy, and the incident has been called the "Saturday Night Massacre," parodying the name of the Watergate incident of the same name. Three of the few--and maybe the only ones--who survived the SNM were Jennine Babo, Dick Ensslen (Drake), and Ray Thorne (FDR), plus Sandy of course. Annie wasn't much of a problem, since Sarah was almost 15 and was outgrowing the part anyway. After having three Annies in a row who outgrew the role at 14 after only one year in the role, Martin decided to go "really young" this time in the hope that the new Annie would be around a while. So he got Allison Smith when she was 10, and the ploy worked, as she played for two and a half years for over 1,000 performances.