The golden rule here to remember is don't change the url. This can have a detrimental effect on your website.
If you do want to change your url rather look into publishing a new post with the url you want and redirecting your old url to the new one instead.
Why do I say don't change your url?
Simply because as soon as you first published that content that url went off into cyber space!
Do you have a blog broadcast on your newsletter? It went out to all your subscribers!
Did you share it socially? Did anyone else share it socially?
Did you link internally to that url? Perhaps someone else linked externally to that url?
So if you have a link to that url anywhere Google will know it is a broken link and that will affect your SEO negatively.
You may also have someone that clicks on your broken link. Not only is it irritating and look bad for your prospective visitor, it also means that you are losing out of traffic!