In GitLab 15.0, you can no longer change an instance (shared) runner to a project (specific) runner.
Users often accidentally change instance runners to project runners, and they're unable to change them back. GitLab does not allow you to change a project runner to a shared runner because of the security implications. A runner meant for one project could be set to run jobs for an entire instance.
Administrators who need to add runners for multiple projects can register a runner for one project, then go to the Admin view and choose additional projects.