However, you can keep importing transactions under new "buffer" account, then select multiple transactions, right click on them, select "Cut transactions", then switch to required account, right click on the transactions grid and select "Paste transactions". I prefer this method, since it shows you just imported transactions separately and then you cut and paste under required account.
You may also import transactions directly under existing account. Please regenerate the QFX file if it was already imported.
If existing account is not listed on the import screen, it means it is already "online linked". To remove "online link", edit the account, click on the Online Services link, select "Remove online services link" and save the changes. Now it will be listed on the import screen.
If you already imported under new account, you need to "unlink" it as well.