--Migration
UPDATE T_INTEZMENYADATOK_OSSZES 
  SET 
     C_JUTTATASHATARNAP = DATEFROMPARTS(DATEPART(year, C_JUTTATASHATARNAP), DATEPART(month, C_JUTTATASHATARNAP), 15)
    ,SERIAL = SERIAL + 1
    ,MODIFIER = 0
    ,LASTCHANGED = GETDATE()
WHERE C_JUTTATASHATARNAP IS NOT NULL