Can the company’s tax number be changed only on new sales invoices?

Yes, this can be done by following these steps:

1. Open the old sales invoice template
   Keep the old company tax number fixed within the template, then unassign it as the default template.
2. Add the new tax number
   Go to Account Settings and add the new company tax number.
3. Create a new invoice template
   Design a new sales invoice template, add the new tax number to it, and set it as the default template for issuing new invoices.

In this way, previous invoices will remain linked to the old tax number, while new invoices will be issued using the template that contains the new tax number.