Instead, Quicken will create a new account. Contacted Quicken, however their support was clearly having trouble understanding the question and they This appears to be a mis-statement as it's really only enforced when Exporting data. After adding the following four lines (documented here) to the top.
Create New File. Select New Quicken Account. Add Account.
Select Brokerage under Primary Accounts. Add Brokerage Account.
Type American Funds in the window. Click Next. Add Brokerage Account (Continued). Enter your Social Security number and americanfunds.com password.
Click Connect. Note: You can enter your fund and account number separated by a comma instead of your Social Security number (e.g., 8). Accounts Added. Review the summary of accounts and transactions added to Quicken. Click Finish.
If you chose a Mac instead of a PC when you purchased a new computer, you may need to make some adjustments to import your files. If you want to transfer your Quicken files from your PC to your new Mac, you need to prepare the old files for that process. The Mac version of Quicken offers a different set of options and abilities than the PC version but Quicken does support exporting your PC files so you can import them on a Mac. Quicken supports transferring your accounts, securities, transactions and categories to Mac.