(My English isn't perfect. Sorry:-)
I wouldn't change the titles and the url structure at the first place.
It is a wrongful approach unless you delete the site and start from scratch. .
It is risky to make such changes, even though the site is over-optimized with its title tags and its url structure, .
This is because Google algo has already defined the keywords in which it sends traffic to the site.
Making such major changes, trigger a red light so the site placed at the bottom of ranking.
What I would do, is to -
1. Start looking very carefully to the redirection profile of the site and cleaning irrelevant 301 and urls.
It could be a good idea to remove any historical redirection no matter how many external links still pointing to the old urls.
2. See the site layout and segment its sectors and sections. Try to understand the way the internal links are formed. Does one section link to another? Could be that one section is way weaker than the other?
I'd make a separation. So that each sector will play on its own.
I have more ideas but this is a start.