You must keep Blog post URL same - Because your old blog post already indexed by different search engines and maybe shared on various social media sites. So if you change the Blog Post URL then when a user click on the link from search Engine result page and social media site like Facebook, twitter, Pinterest etc. then they will land directly into error 404 page. As a result, you may lose referral traffic and pageviews as well.
Source : http://www.bloggerspice.com/2017/03/change-blog-post-date-on-blogger.html