Hi David,
Of the people I have traced in Australia who changed their surnames by far the majority were escaping money they owed to someone. Men who married again after leaving a wife and family behind in the UK didn't usually bother to change their names. Apparently they were more concerned that owing money was more likely to be followed up, and wives and families left behind would have few resources to track them down. But that is a generalisation, and I have no idea why your James would bother to change his name. It seems that his family in the IOM didn't know he had done so, with his father naming him the same as his other children (without the "Parker") in his will. He was left a considerable sum of money for those days, so you would expect his family to have bailed him out if he was in trouble. I don't think the "Parker" has any significance, but was just a common name he chose.
Sue