In the same way a cobbler's children have no shoes, I had an old computer sitting on my network running CentOS 8.3. A frame of reference since you're reading this in the future, this is being written 2 months after CentOS 8 went EOL, and a little over a year since 8.4 was released. It wasn't doing anything other than holding harddrives, and it didn't have a network cable plugged into it for a while, so it wasn't much of a threat.

Necessary disclaimer, I hope you're not doing this on production.

Repo Error

It isn't too hard to find information on how to migrate CentOS 8 to AppStream such as this (article at Linode)[https://www.linode.com/docs/guides/migrate-from-centos-8-to-centos-stream/], but if you're reading it now, you're so far behind you're going to get an error trying to get follow the steps.

[root@core2 dan]# dnf info centos-release-stream
CentOS Linux 8 - AppStream                                                             115  B/s |  38  B     00:00
Error: Failed to download metadata for repo 'appstream': Cannot prepare internal mirrorlist: No URLs in mirrorlist

This is caused by the repos no longer being available because of how out of date it is.

Sed Fix

A few quick seds on the repo files will fix it right up. These will change the mirror.centos.org URL to vault.centos.org to allow pulling the required packages straight from the CentOS vault, rather than checking non-existant mirrors.

[root@core2 dan]# sed -i 's/mirror\.centos/vault\.centos/g' CentOS*.repo
[root@core2 dan]# sed -i 's/^mirrorlist/#mirrorlist/g' CentOS*.repo
[root@core2 dan]# sed -i 's/^#baseurl/baseurl/g' CentOS*.repo

Once that is complete you can continue on with the migration. These repo files will be modified to append .rpmsave and will be ignored by DNF when adding the stream repos.