In the page Upgrade from Jira Server to Jira Data Center for 9.13 and 9.14 we have the following text:

To upgrade from Jira Server to Jira Data Center, you will need a Data Center license. You can either purchase a full Data Center license or get a free trial license for 30 days. When your 30-day trial finishes, you won't lose any data you've created. You'll have the option to either purchase a Data Center license or carry on using Jira Data Center in read-only mode. If you decide Jira Data Center is not for you, you can easily revert to your existing Server license.

One of the issues is that Server licenses cannot be applied to 9.13 or 9.14.
The other issue is that this is present in the documentation and customer with an expired Server license, will not be able to rollback.