The FM Books Connector Online plug-in uses the user account that is connected during the Begin Session process, which defines what the plug-in has access to (such as customer information, invoices, bank data, etc.). Since the previous user "XXXXX" is no longer part of the business, the plug-in could reconnect the session to log in with the new user "XXXXX" so that it will use the new user's access privileges for exchanging data. Technically speaking, the plug-in does not modify any settings directly, so it is unclear as to what settings will be "forgotten" when switching the user, but realistically it should not matter. If necessary, you can log into the QuickBooks Online company preferences and review the user account settings for the XXXXX and XXXXX accounts, screenshot the details to preserve what is currently set, and then make the switch via the plug-in to use the new account. That way you can recover the settings if need be.