yes you can edit an old invoice and save it

you should not be entering estimated invoices though, an invoice is a sale and affects your P&L and VAT

use an estimate, when the charges are final, create an invoice from the estimate

Resolution for 'charges are final issue: 'Can you edit an invoice in a prior month after it has been reconciled?' available: Yes.

The charges are final issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the charges are final issue yourself by looking for a resolution described in the article. If it is a complex charges are final issue or you are unable to solve the issue, you may contact us by clicking here or by using other charges are final support options.

For more information about resolving issues related to charges are final in Quickbooks/QBO or to get support for the issue 'Can you edit an invoice in a prior month after it has been reconciled?', click the link below: