Try using the information in the Back up indexed data topic in the official documentation. It includes the CLI command for manually rolling hot to warm, as well as information about why you might not want to.
The hot to warm rolling commands in that documentation works. I am running Splunk in a docker container and need to run a backup job to put databases in a S3 bucket. There seems a lot pros and cons to consider if and when a hot db should be included in the backup plan. The documentation It's very helpful. Thanks!