DECLARE @X TINYINT = 0

;WITH SampleTable AS (
SELECT SOMECOLUMNS = 'SOMECOLUMNS', COLUMN1 = 'COLUMN1', COLUMN2 = 'COLUMN2', COLUMN3 = 'COLUMN3',
COLUMN4 = 'COLUMN4', COLUMN5 = 'COLUMN5', COLUMN6 = 'COLUMN6'
)
SELECT SOMECOLUMNS, x.*
FROM SampleTable
CROSS APPLY (
	SELECT COLUMN1 = 'COLUMN1', COLUMN2 = 'COLUMN2', COLUMN3 = 'COLUMN3' WHERE @X = 1
	UNION ALL
	SELECT COLUMN4, COLUMN5, COLUMN6 WHERE @X <> 1 
) x

If @x=1
	Select column1, column2, column3 from MyTable
Else
	Select column4, column5, column6 from MyTable

select case when @x = 1 THEN column1 else column4 end  as col1,
case when @x = 1 THEN column2 else column5 end  as col2,
case when @x = 1 THEN column3 else column6 end  as col3
from MyTable

DECLARE @X bit, @strSQL nvarchar(2000)
SET @X = 1

SET @strSQL =' SELECT ' + CASE WHEN @X = 1 THEN ' COLUMN1,COLUMN2,COLUMN3'
                           ELSE ' COLUMN4,COLUMN5,COLUMN6' END 
             + ' FROM TABLE1 '              

EXEC (@strSQL)