Downgrade Windows Server 2008 R2 Standard to non-R2
Does anyone know how to downgrade Windows Server 2008 R2 Standard to non-R2. Apparently the 2 uses diffenret licence keys. Initially I installed the R2 version but later I realise that I am only entitled/have the non-R2 standard licence key. I cannot activate the R2 licence with the non-R2 key. Is there a way for me to downgrade the R2 to non-R2 so that my licence key is accepted, other than reinstalling it from scratch again?