It has already been announced that Broadchurch star Olivia Colman is set to take over Foy's role as Queen Elizabeth in seasons three and four.
In seasons 1 and 2 - the latter of which premiered on Netflix in December - Princess Margaret is portrayed by Vanessa Kirby.
This post contains mild spoilers for The Crown's second season.
Kirby told BAZAAR.com she relished playing Margaret's "dark nights of the soul" in Season 2, and we're sure to see more of that from Margaret in coming episodes.
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The actress, known for Merchant Ivory films like A Room With A View and frequent collaborations with former partner Tim Burton, is "all but confirmed" play the role of Queen Elizabeth II's younger sister, according to the Evening Standard. Morgan previously said he meant to replace key cast members in later seasons, to better portray the characters as they aged.
Bonham Carter has had plenty of experience playing British royals previously.
The Crown showrunner Peter Morgan always planned to re-cast the series after its first two seasons, telling Variety back in 2016, "You can't ask someone to act middle-aged".
Carter's most notable roles include an Oscar-nominated turn in Tom Hooper's The King's Speech opposite Colin Firth and an unforgettable character in David Fincher's Fight Club opposite Brad Pitt and Edward Norton. As Chris writes in his review, it's "an old fashioned, admirably episodic costume drama with superb performances, intricate direction, and enough simmering, bottled-up righteous fury to keep you hooked from beginning to end (give or take an episode or two)". The first two seasons covered the first 15 or so years of the queen's reign in the 1950s and 1960s.