If I understand you correctly, you want to access the old posts only with the old permalink structure (e.g. ?p=123), and the new posts only with the new structure(e.g. /2009/08/name-of-article).
I'm afraid this is not possible without rewriting the Wordpress code (see short explanation at the end of my answer).
However, I think that Scott Yang's permalink redirect plugin (scroll down to FAQ) or Dean Lee's permalink migration plugin could be a solution.
When Wordpress generates a link (e.g. for a new article, for "continue reading" links or for search results), it will use the new permalink structure, even for the old posts (which sadly isn't exachtly what you intended). If someone however uses an old permalink, it will be rewritten on the fly and the user (or SE) will be redirected to the correct page.
This 301 redirect should keep your page rank.
Explanation why having different permalink structures for old and new posts in parallel would mean changing Wordpress code:
For each post, Wordpress stores an unique ID (e.g. "123") and not necessarily unique post name (e.g. "name-of-article") in the database.
When an URL with a permalink like /2009/08/name-of-article is accessed, Wordpress translates this according to the currently configured permalink structure. If it constructs a link, it also does this according to the currently configured permalink rule.
You would need to add code toe Wordpress so that it uses two different rules depening on the age of the article.