Hello,
I'm Alex, run a small digital agency, we are currently mastering at WordPress, PHP and RoR.
URL things are usually complicated, especially if there are lots of live pages already. As soon as URL changes are made, less problems will occur later.
We will change URL's on your given instructions and will make a script which will check if there are any URLs which goes to 404, if yes, then another script will redirect bad links to new, working links.
Let me know the details so that I could confirm the budget.
Thanks!
Alex