I called MH again and explained the situation. I was told that the previous agent was incorrect with the +/- 7 day statement.
The booking was in O class and there was an O class available in late April, returning mid August (which is way outside the +/- 7 day window) and she said the fare difference will be waived for the first re-booking which in this case was the first one and therefore a new ticket has been re-issued for the dates my parents wanted and that was a great outcome.