We are trying to downgrade our 2019 datacenter eval to 2019 standard but we are having difficulties.
This also is not possible to downgrade the edition from DataCenter to Standard, so yes a clean install is necessary.
Our reseller can't provide us with licenses below 2022, he says that Microsoft doesn't sell them anymore.
That's correct, they don't sell Server 2019, but there are downgrade rights. Used to be one could call the activation center but these days calling microsoft is near impossible which is why I suggested you get the reseller to do this for you.
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