I know I *could* (and probably should) do a redirect; I’m just wondering if Google eventually does start crawling the new slug and removing the old one from its search results. I’m playing around with some old, very low-traffic blog posts on a site I run, and I haven’t been able to find any clear answers about this.
To get the old one to stop showing in the SERPs I would either redirect or noindex the old page. Otherwise it might take a while for Google to index the new page and determine that to be the main page for the subject-matter/ranking kw’s.
Plus if you redirect, to my understanding, it should pass the PageRank (less about 15% according to Google) to the new URL.