DECLARE @MAX_COUNT INT;
DECLARE @COUNT INT;
DECLARE @INC INT;

SET @COUNT = 0;
SET @INC = 50000;

SELECT @MAX_COUNT=MAX(ID) FROM  dbo.MYTABLE(NOLOCK)
WHILE @COUNT <= @MAX_COUNT
BEGIN

    UPDATE dbo.MYTABLE
    SET NEW_NAME = REPLACE(NAME,' X','Y'))
    WHERE ID > @COUNT AND ID<=( @COUNT + @INC)

    SET @COUNT = (@COUNT + @INC)
END