When you update the Default Email Subject and Default Email Message under Payment Center Settings > Customize Invoice, the changes will apply to new Invoices and Recurring Invoices moving forward.
For Invoices and Recurring Invoices that already exist, the email subject and message will need to be updated separately.
Updating existing records
You can update the following fields directly on the Invoice or Recurring Invoice record:
Default Email Subject
Default Email Message
If you need to update a large number of existing records, you can perform a mass update using your preferred Salesforce data tool, such as Data Loader, Salesforce Inspector etc.,
Simply update the Default Email Subject and Default Email Message fields for the applicable Invoice or Recurring Invoice records and upload the changes.