You will not lose your rankings any way. This is the same situation we have had on our forums previously, when we have had both dynamic pages and mod_rewritten. The only thing you need to know:
- if you have an URL like index.php?action=vthread&forum=2&topic=3690
(this thread), this page is shown, but not indexed, 'cause there are NOINDEX, NOFOLLOW tags.
- if your URL is like 2_3960_0.html (mod_rewritten), it's both shown and indexed.
So, when you have turned mod_rewrite on:
- your old-type URLs will be available for some time, but as soon Google re-visits them, as soon it will delete them from his cache;
- your new-type URLs will be indexed by Google from scratch.
We don't think some redirects are necessary. This is the same situation happened to us months ago.