Set the Account RecordTypeId field to the Person Account RecordTypeId. Be aware you are only allowed to specify the RecordTypeId field when converting the account, and only one Contact must be associated the Account when converting.
Manipulate the data such that the following conditions are met:
a. For each intended Person Account, one (and only one) Contact record
exists under the Account record.
b. The Account and Contact have the same record owner.
c. The Account and Contact have the same currency values (if
d. The Parent Account field on the Account and the Reports To field of
the Contact are both blank.
e. The Account is not set as the Parent Account of any other Account
f. The Contact is not set as the Reports To of any other Contact
g. The Account and Contact have not been enabled as Partner/Customer
Using the data migration tool of your choice (i.e. something like the
Data Loader), change the record type of the target Account to the
desired Person Account record type. The detailed steps are listed
here. *Please note that this cannot be done with Workbench, as it will
return the error: "Cannot Specify Any Additional Fields When Marrying
Or Separating A Person-Account INVALID_FIELD_FOR_INSERT_UPDATE"