Now I do not know this for a fact but my understanding is that the switch does not have a transformer so it cannot step the voltage up or down. It can just relay what it get from the power supply.
The above is correct; the PoE output is whatever voltage you feed it.
You can run the hEX PoE on a 24 V PSU and it will give 24V PoE, but then will not be able to work with other devices which require af/at PoE. That requires a 48V+ PSU. As long as you don’t need to do that, 24V is fine.