Changing site structure

Is is easy to change my sites URL structure? I’ve been running the site for some time and have hundreds of posts now but I’m not happy.

For example all categories are currently located like this:

https://www.mysite.com/category/news/
https://www.mysite.com/category/articles/

By contrast my pages are:

https://www.mysite.com/about/
https://www.mysite.com/contact/

I want to remove the ‘category’ part of the URL so I have:

https://www.mysite.com/news/
https://www.mysite.com/articles/

Also, an individual post will appear like this:

https://www.mysite.com/2009/08/08/news/post-title/

I want to remove the date part so it becomes:

https://www.mysite.com/news/post-title/

This would make everything much cleaner. If it is possible to do this, how? Also if I do it, how will I stop links leading to my site going to dead pages?

I don’t want to damage my Google search ranking by having loads of 404 pages. But I just can’t help think they will be even better with a more sensible structure.