CREATE TABLE #Test (a VARCHAR(30))

INSERT INTO #Test (a) SELECT REPLICATE('A', 29)

ALTER TABLE #Test

ALTER COLUMN a VARCHAR (29)

--this works fine

DROP TABLE #Test

GO

CREATE TABLE #Test (a VARCHAR(30))

INSERT INTO #Test (a) SELECT REPLICATE('A', 30)

ALTER TABLE #Test

ALTER COLUMN a VARCHAR (29)

--Throws error message because it would change the data

GO

--You can figure out which rows will return an error with a query like this one :

--SELECT * WHERE Column <> Convert(NewDataType, Column)

SELECT * FROM #Test WHERE a <> CONVERT(VARCHAR(29), a)

DROP TABLE #Test