When a site has been public for a period of time it has many indexed URLs by search engines and bookmarks etc.
If you have created a new site and changing the page structure or behavior of the URL logic you will end-up with alot of requests that is not handled by the new site. Users will get 404 pages and in a user perspective it will not be a good user experience.
So the first step you need to do is to add functionalty to handle the old URLs. Some kind of functionality to be able to redirect users from one old URL to the new URL.
When that is implemented it is still hard work to test all URLs if they are handled or not. With Experiortools you will be able to do that test and get a simple report what URLs you are handling and not. You will be able to re-run the test and see that you are making progress to handle old URLs.