1) Drop the existing clustered index first (IX_TableX_FieldB):
DROP INDEX TableX.IX_TableX_FieldB
2) Create a (temporary) UNIQUE constraint on the unique fields referenced in the primary key
ALTER TABLE TableX
ADD CONSTRAINT UQ_TableX UNIQUE(FieldA)
3) Drop the PRIMARY KEY
ALTER TABLE TableX
DROP CONSTRAINT PK_TableX
4) Recreate the PRIMARY KEY as CLUSTERED
ALTER TABLE TableX
ADD CONSTRAINT PK_TableX PRIMARY KEY CLUSTERED(FieldA)
5) Drop the temporary UNIQUE constraint
ALTER TABLE TableX
DROP CONSTRAINT UQ_TableX
6) Add the IX_TableX_FieldB back on as NONCLUSTERED
CREATE NONCLUSTERED INDEX IX_TableX_FieldB ON TableX(FieldB)