The world of Gilead is one with specifically defined castes, born of a theocratic society that finds itself battling a major worldwide fertility crisis. In author Margaret Atwood's original The Handmaid's Tale novel, these castes include (but are not limited to) Commanders, Angels, Guardians, Wives (plus Econowives), Marthas, Handmaids, Jezebels, and Aunts. As far as readers see, these castes are immutable; once you are assigned, that's your lot. But the TV series has suggested these are more fluid than one might think. For example, the possibility that Serena Joy will become a Handmaid is no longer as unthinkable as it once seemed.
Warning: Spoilers for The Handmaid's Tale Season 4, Episode 9 follow. The series first revealed that high-ranking women could be demoted in Season 2, when it introduced the Unwomen in the Colonies as a Wife sent there after her affair with her chauffeur. But the real caste that matters is that of the Handmaid. In a world where 75% of the population is infertile, the 25% of women who can carry healthy children to term are too important to allow to be anything else. At least, that's part of Gilead's entire premise.
For all that the Aunts make noises about June being an adulterer or Emily being a “gender traitor,” the real thing that defines them is their fertility. That's why the Wife in Season 2 didn’t go into Handmaid service after her crimes. As a woman deemed infertile, in Gilead’s eyes, there's no reason to keep her around.
In short, anyone who proves their ability to have a baby is subject to being assigned to Handmaid status. That's why Esther Keyes has been reassigned to Handmaid status.
And that's why Serena Joy can never go back to being a Wife.
The Putnams' visit to Canada in Episode 5 was super telling of how Gilead now sees the Waterfords. Naomi wasn’t really there to bring presents. She was there to gain Serena Joy's trust and use that to gain custody of her unborn child. She was also there to sweet talk Serena into returning to Gilead in hopes she's foolish enough to believe she'll be greeted as a baby-making saint, rather than a traitorous soon-to-be Handmaid. Warren, meanwhile, was there to pretend this trip wasn’t all about getting their hands on Serena and her child. But his talk with Fred made it clear Gilead has zero interest in the Commander's fate.
But Serena Joy is not a fool. She may have once mistakenly believed that Gilead would have a place for smart women. But her missing pinkie is a lesson Serena won't need to learn a third time. She knows Naomi is lying, that there is no way she won't be arrested for crimes against Gilead the second she steps foot back on her home soil. Moreover, she's proven to be able to bear a child now. And there's only one dress the fertile ones wear: red.
The Handmaid’s Tale continues Wednesdays on Hulu.