Hi DragutinKrstaji-9179, Same happened to me. I had a terrible time calling Microsoft, waiting long, transferred from one department to another and at the end calls cut abruptly. Vendor tells me to check with Microsoft, and Microsoft tells me check with vendor. At the end one Microsoft guy listened to me carefully and replied properly.
The answer is, Microsoft does not provide downgrade key for OEM license purchased from re-sellers. If your OEM license came with hardware, there is a possibility of downgrade kit provided by the manufacturer. So I had to upgrade the server from Windows 2016 Standard to Windows 2019. The upgrade was smooth, all of the applications kept working fine. But yes, it is a risk, your applications may stop working.
It was a cheap tactic on vendor side to sell the license without telling that is not down gradable. There is a document which tells downgrade rights, but it is not as clear as I mentioned above after going through the experience. Good Luck