hazmat - 5:59 am on Jan 12, 2012 (gmt 0)
More of a framework, but codeigniter allowed me to keep the url structure roughly in-tact. Page structure varied to some degree, and navigation definitely did, as we did a redesign at the same time. Totally worth the effort as the site is so much more maintainable now, like a breath of fresh air.
If a CMS is required there are quite a few other options. I'd personally stay away from joomla, or drupal.