I was wondering if it is possible to just overwrite a new provided license file over the old one, or is a reinstall of the deadline server (as stated in the support mail) absolutely necessary?

Thanks!

Both methods are supported, as explained in the documentation:
docs.thinkboxsoftware.com/produ … ewlic.html

We recommend reinstalling as it is more authomated and ensures you are running the latest version of the License Server. But overwriting the file and restarting the license server also works. (Some people restart the license server machine to be absolutely sure it will work right )