Will Changing the Customer Display Name for QBO Update Accounts that Were Already Synced?

No, changing your Customer Display Name Format on the QBO configuration page in the Recurly Admin Console will not affect existing accounts that have already been synced.

These existing accounts will continue to display the old Customer Display Name Format. Only new accounts that are synced over after the change will use the new format.

For example, if you originally used Email as the Display Name and then switched to First & Last Name, all accounts synced prior to the change will still display the email. Accounts created after the change will display the first and last name.