Hi Stuart,
Thank you for reaching out with your concern about losing your transactions. Let me point you in the right direction so that you can still have them even if you disconnect your personal account.
You can still disconnect your personal account without necessarily losing your transactions from it. All you want to do is export them to Excel before disconnecting your account and then import them. This process will create another account called Cash where they will be grouped into.
Export transactions to Excel:
- Go to Transactions.
- In the Account filter above TRANSACTION, select your personal account.
- Click the Download icon on the right.

After exporting the file, import the transactions using the steps in this article: Manually import transactions into QuickBooks Self-Employed. However, if you're unable to import them because the file is not in a CSV format or because it has too many columns, be sure it has the Date, Description, and Amount columns. Then, you can save it as CSV (Comma delimited or Comma Separated Values), and try importing again.
On the other hand, if you have an existing Cash account type, you can directly import older transactions to it, as shown below. A Cash account type is automatically created when you enter transaction/s while the Account filter is set to All.

You can also check about deleting bank accounts. It contains steps on how you can disconnect your personal account using a computer browser or your QBSE mobile app.
Let me know if you have questions as you go through the steps. It's my pleasure to ensure that you're able to keep track of your business transactions. Have a lovely evening!