I usually set the new Gmail to retrieve email by POP/IMAP from old one, and have a filter defined in the old one:

    Matches: from:(*)
    Do this: Never send it to Spam, Exclude from SmartLabel categorization

In this way, every single email is forwarded to the new account and filters in the new one deals with spams or whatever. But you can always conveniently check mistakenly spammed emails in your new account.