But I have a question in regards to the historic Invoices and accounts. Do you have to maintain old accountancy software or can this be exported and imported?
Be sure to adjust your trial balance before importing it to account for for these outstanding invoices so you do not double up on the values. in the debtors and creditors etc.
If you search Xero help, there are several articles on what you need to do / export if you are leaving Xero, including exporting the TB, debtors and creditors, etc. You may wish to ask your accountant to do this, as they will ensure everything gets moves across correctly.