大致先更换vault源，然后执行切换升级。

# CentOS 8 
minorver=8.5.2111
#mirrors=https://mirrors.aliyun.com/centos-vault
mirrors=http://ftp.iij.ad.jp/pub/linux/centos-vault
sudo sed -e "s|^mirrorlist=|#mirrorlist=|g" \
         -e "s|^#baseurl=http://mirror.centos.org/\$contentdir/\$releasever|baseurl=$mirrors/$minorver|g" \
         -i.bak \
         /etc/yum.repos.d/CentOS-*.repo

然后dnf更新系统。

dnf install centos-release-stream -y
dnf swap centos-linux-repos centos-stream-repos -y
dnf distro-sync -y

Centos8 stream更换vault源

mirrors=http://ftp.iij.ad.jp/pub/linux/centos-vault/8-stream/
sed -e "s|^mirrorlist=|#mirrorlist=|g" -e "s|#baseurl=http://mirror.centos.org/\$contentdir/\$stream/|baseurl=$mirrors|" CentOS-Stream-AppStream.repo    \
 -i.bak /etc/yum.repos.d/CentOS-*.repo

可能有一些第三方包不能升级， 也有可能遇上centos-release-stream安装失败的情况。