Moving from one WP site to a new WP site incrementally

Have you thought about doing this with per-url redirects? I found something promising here: http://www.phpin24.co.za/apache/apache-change-documentroot-for-a-specific-url/

It seems to boil down to this (copied from the linked page):

<VirtualHost *:80>
    ServerName www.phpin24.co.za
    DocumentRoot /var/www/html/phpin24

    AliasMatch ^/enquire-now/thankyou/(.*) /var/www/html/other-code/app/webroot/$1
    AliasMatch ^/enquire-now/(.*) /var/www/html/other-code/app/webroot/$1

    ErrorLog /var/log/httpd/phpin24/phpin24_error_log
    CustomLog /var/log/httpd/phpin24/phpin24_access_log combined
    <Directory /var/www/html/phpin24/>
                Options FollowSymlinks
                AllowOverride All
                FileETag none
    </Directory>
</VirtualHost>