Error when using WITH (DROP_EXISTING = ON)
This can happen when the current primary key is nonclustered, and there is also a clustered index:

CREATE TABLE dbo.MyTable
(
    MyTableID integer NOT NULL,
    RowStatus tinyint NOT NULL,
    SomeOtherKey integer,

    CONSTRAINT [PK_MyTable]
        PRIMARY KEY NONCLUSTERED (MyTableID, RowStatus),

    CONSTRAINT [CUQ_MyTable]
        UNIQUE CLUSTERED (SomeOtherKey)
);




CREATE Unique CLUSTERED INDEX PK_MyTable 
    ON dbo.MyTable (MyTableID, RowStatus)  
    WITH (DROP_EXISTING = ON);



  Msg 1902, Level 16, State 3, Line 14
  Cannot create more than one clustered index on table 'dbo.MyTable'. Drop the existing clustered index 'CUQ_MyTable' before creating another.


It is valid to change a primary key from nonclustered to clustered using DROP_EXISTING syntax:

CREATE TABLE dbo.MyTable
(
    MyTableID integer NOT NULL,
    RowStatus tinyint NOT NULL,
    SomeOtherKey integer,

    CONSTRAINT [PK_MyTable]
        PRIMARY KEY NONCLUSTERED (MyTableID, RowStatus),
);

CREATE Unique CLUSTERED INDEX PK_MyTable 
    ON dbo.MyTable (MyTableID, RowStatus)  
    WITH (DROP_EXISTING = ON);