Debicki, an Australian actor who starred in Tenet and The Night Manager, takes over the role from Emma Corrin for the final two seasons of the royal drama.
West, who is known for his roles in The Wire and The Affair, takes over the role from Josh O’Connor for seasons 5 and 6.
The show's fourth season ended with an already unhappy Diana who was struggling to fit in with the royal family while grappling with an eating disorder and her troubled marriage to Charles. Both are shown having extramarital affairs.
The last two seasons of The Crown are expected to depict Charles and Diana's divorce as well as the public revelation of his affair with Camilla Parker Bowles. The show is also expected to cover Diana's tragic death as well as the deaths of the Queen Mother and Princess Margaret, the queen's sister.
Last month, Netflix released the first look of Harry Potter actor Imelda Staunton as Queen Elizabeth II.
Game of Thrones actor Jonathan Pryce will portray Prince Philip, taking over from Tobias Menzies, while Lesley Manville will portray Margaret, who was previously played by Helena Bonham Carter.
This year, The Crown, along with The Mandalorian, leads the Emmy nominations with 24 nods each, including one for Outstanding Drama Series.
Season 5 of The Crown is expected to air in 2022.