lucy24 - 1:30 am on Apr 24, 2013 (gmt 0)
Are you sure you don't want the Apache forum? ;)
Well, no matter, because the person who is going to answer your question is on vacation. But he will be back before your site is 100% converted.
You have probably already figured out that the most important question is how to deal with the Great Divide. On one side: pages that have been converted and are now going to be handled by your CMS-- here Drupal, though it could be anything. On the other side: pages that are still in static-html form so they have to be ignored by all rewriting. You don't want 300 individual RewriteRules clogging up your htaccess. Sure, it's physically possible-- it's just not pretty.
It would be superb if you could make the change one directory at a time. Then you've got an obvious, visible identifier that all your RewriteRules can look for: this group of URLs go to Door #1, that group go to Door #2.
Conversely, it would be decidedly un-fabulous if you had to rewrite all requests to a transitional php script which then picks out which pages go to the CMS and which ones are allowed to get served up as-is. As with the 300-rewrite-rules scenario, a double-rewrite solution is doable but not pretty.
Does it absolutely have to be done in installments? Is there any possibility of doing the work offline and unveiling the whole new improved version after everything is done? Do you have a lot of dynamic content-- I mean truly dynamic, user-generated stuff, not just php doodahs? Will the change be clearly visible to the user, or is it primarily behind the scenes?