Upgrading can be possible theoretically, but not sure if Microsoft really wants us to do it because whenever I try it, I get stuck. Sometimes it doesn't respond for about 1 hour and then accepts the command. Sometimes it is 2 hours or more and sometimes it doesn't work at all. So using DISM cannot be advised to anyone; there's no guarantee that it will work. And most of us use Windows Server for business applications and in such an environment, we need to rely on something that's sure to work.
So my advice: DISM doesn't work anymore with edition change as it used to on Windows Server 2012. If you're looking for an edition change, reinstall is the way to go. Or don't install the evaluation in the first place; always download media from Microsoft VLSC or use the retail disk.