There is no official guide to migrate from Rocky Linux to CentOS Stream, however, it can be achieved by using official CentOS to CentOS Stream migration guide

1. Install centos-release-stream package

dnf install https://vault.centos.org/centos/8/extras/x86_64/os/Packages/centos-release-stream-8.1-1.1911.0.7.el8.x86_64.rpm

2. The next step is to swap Rocky Linux repositories to CentOS Stream ones. This is done with dnf swap centos-{linux,stream}-repos -y command. However, this command will cause multiple conflicts in Rocky Linux. To solve them:

• copy /etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial file from any Centos Stream system into Rocky Linux
• remove conflicting packages:

  rpm -e --nodeps rocky-repos
  rpm -e --nodeps rocky-release
  

• complete installing CentOS Stream repositories

  dnf swap centos-{linux,stream}-repos -y
  

3. Switch to CentOS Stream

dnf distro-sync -y

4. Verify installation and reboot the server:

cat /etc/centos-release
reboot