No, you should not upgrade, always wipe & load. There might be scenarios where you would like this to work, but it does not. Upgrade on the Server preview version has been blocked. The way to solve this issue is to extract the ISO image, open the source folder and delete the file “noupgrade.txt” and then run setup.exe from the folder.
I have done this on 4 Hyper-V host machines so far and it worked for me. The reason was that I did not want to export/import all the machines just to test new Hyper-V stuff.
However, It is not supported, not recommended.