Emma Rogan has become the latest Sinn Féin MLA to be deselected as an election candidate.
Following sweeping changes in Foyle, the party has also made changes in South Down, replacing a sitting MLA with a councillor.
It is understood Ms Rogan put her name forward to stand again in the constituency but was not chosen as a candidate following a selection convention.
Councillor Cathy Mason, who sits on Newry, Mourne and Down District Council, was chosen ahead of Ms Rogan.
Sitting MLA Sinéad Ennis will stand again in the South Down constituency.
It is understood the selection convention was held a number of weeks ago.
Ms Rogan (34) has served as an MLA since June 2017. She was co-opted to replace Chris Hazzard after he clinched the South Down Westminster seat.
A respected Loughinisland massacre campaigner, her father Adrian was killed in the 1994 loyalist gun attack.
A Sinn Féin spokesman said: "Following a recent selection convention held in the South Down constituency the membership elected Sinéad Ennis and Cathy Mason to stand for the party in the next Assembly elections."
Ms Mason, from Drumaness, was first elected for Slieve Croob in the May 2019 council election.
The change in South Down comes amid controversial developments within Sinn Féin in Foyle.
MLAs Martina Anderson and Karen Mullan announced earlier this month they will not contest the next assembly election.
The pair were asked not to stand again following a run of poor election results in Derry.
Ms Anderson, a former MEP and junior minister, is a veteran republican activist and former IRA member who served more than a decade in prison.
Her family attacked Sinn Féin's decision to deselect her following a review of the party's structure in Foyle.
Ms Anderson's sister Sharon Burke wrote on Facebook: "The British could not do to our Martina what her comrades and friends have done".
Changes within Sinn Féin are a further indication that the north's main parties are preparing for an early assembly election.
Once Arlene Foster leaves her post at the end of June, the DUP must put forward its pick for first minister.
Sinn Féin is not expected to back the DUP's choice without major concessions, primarily around the Irish Language Act.
Without Sinn Féin agreement, the secretary of state must call an election, most likely for the autumn.
