It's a good thing to keep your pages updated. It's a good thing to update the date every time you make big changes.

Google sees big changes as having the latest information and will use an old post as a new post when it comes to publishing frequency.

What you seriously do NOT want to do is change your permalinks.

Maybe you chose some URLs that you liked at the time, but they don't seem to be good keyword-rich names that will drive traffic.

Google looks at your content MUCH more than your permalink.

Yes, you can use it as a tiny little additive when it comes to the entire scope of what your content is about, but don't obsess over it. If your content is already indexed, it is somewhat of a complicated process to change the permalink without Google thinking you are posting duplicate content.

Duplicate content will obliterate your ranking. It will take months to recover.

It seriously isn't worth playing with your permalinks after you are indexed.