Downgrading from QuickBooks Online Plus to Simple Start will not delete your accounts or core transaction history, phil95. Your company, customer, vendor, and existing bank transactions remain intact. However, the "loss of data" message refers to specific features and advanced data fields not supported by the lower plan.
One important point to consider based on your workflow is that billable expenses are not supported in Simple Start. This feature requires at least the Essentials plan, so if you regularly allocate billable expenses from bank transactions to invoices, you will lose that functionality after the downgrade.
Before downgrading, please ensure to turn off any features supported by your current plan that aren't available in the Simple Start version to prevent potential issues. For detailed instructions, check out this article: Upgrade or change your subscription.
Please let us know if you need any further assistance.