"Ridurre un supergruppo a un gruppo normale" is meant to indicate that a supergroup is bigger than a normal one. For this reason, it cannot be 'reduced'/made smaller. The meaning of the verb 'ridurre' is closer to 'downsize' than 'trasformare' and it is clear in this context.

This is the literal translation from English to Italian. The term "declassare" is the exact translation of "downgrade" in English. "Declassare" means moving from one class to a lower one.