INVALID_FIELD_FOR_INSERT_UPDATE, Cannot specify any additional fields when marrying or separating a Person-Account: []

When trying to programmatically convert a business account into a person account using Force.com Apex, I'm getting INVALID_FIELD_FOR_INSERT_UPDATE, Cannot specify any additional fields when marrying or separating a Person-Account: []

found this thread at the developer force. The solution goes:

The (nonobvious) solution here is that, when you change the RecordType of the Account, you need to be operating on an Account object that has nothing but its Id defined.

In other words, this does NOT work:

Account a = new Account(Name = 'Test PAConv');
insert a;

Contact c = new Contact(LastName = 'PAConv', AccountId = a.Id);
insert c;

a.RecordTypeId = PA.getRecordTypes()[0].Id;
update a;

It fails with "INVALID_FIELD_FOR_INSERT_UPDATE, Cannot specify any additional fields when marrying or separating a Person-Account []"


However, re-fetching the "a" record with only its Id lets the whole thing work:

Account a = new Account(Name = 'Test PAConv');
insert a;

Contact c = new Contact(LastName = 'PAConv', AccountId = a.Id);
insert c;

// re-fetch a before updating RecordType
a = [select Id from Account where Id = :a.Id];

a.RecordTypeId = PA.getRecordTypes()[0].Id;
update a;

By: https://www.ihance.com/aav2/appexchange.aspx