Maybe I am over-simplifying this as I do not understand your request fully, but could this be as simple as:
UPDATE LoadData
SET
Sequence_Code = dbo.GetNextSequenceCode_TEST ( ID )
FROM LoadData AS ld
WHERE
ld.Sequence_Code IS NULL
AND ld.[Status] = 'A';
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…