If I install a demo license on a production server it affects the current licenses.
If I install a demo license on a production server it affects the current licenses.
I don't think you can.
The usual approach is to temprarily delete/unenroll some production device(s) to release device license(s) for enrolling demo/test devices. After tests/demonstrations, delete such devices and re-enroll back the production device(s) previously deleted.