Maybe, you can use a dynamic query

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)