1. Enable Person Accounts
2. Do a Data Backup.
3. Find the Record Type ID's for the Accounts (now called Business Accounts) and for Person Accounts.
4. Have a .CSV prepared for the new Contacts that you want to Import to salesforce that will become Person Accounts.
5. You must import the new Person Accounts through the Import Wizard. Choose Import Person Accounts.
Note: This will cause duplicates in your system, however since you have the Account ID's of the accounts that you wish to convert to Person Accounts, you can delete those accounts at the very end.
But first you need to associate the new Person Accounts with the existing Opportunities that the Account ID's have been associated with.
6. Run an Export based on the criteria matching your Person Account ID. This will give you all the Person Accounts and will allow you to get their new ID's to run the update.
7. Do a V-Lookup on the "Old Account ID" field with all the Opportunities ID's. This will allow you to see what Opportunities the Accounts were associated with before you created a duplicate Person Account.
8. Update the Opportunities with the new Person Account ID's, replacing the old Account Id's.
9. Check to see in salesforce if your Person Accounts are now associated with the correct Opportunities.
10. Hide the field "Old Account ID" off of the Contact Page Layout, because it is no longer needed.
11. Delete the duplicate accounts by running Data Loader. You already have the ID's you wish to delete because they are located on your spreadsheets in the "Old Account ID" column.