Student permesso → research fellowship in Italy: Prefettura says no Nulla Osta needed?

Hi, I’m a bit confused about the residence permit process in Italy.

I recently completed my PhD and started a research fellowship with a university. I have a research contract and a Convenzione di Accoglienza. My current permit is permesso di soggiorno per studio, and I submitted the renewal application at Poste before it expired.

While the university’s international office was preparing the application, we contacted the Prefettura. After checking, they called back and said I do not need a Nulla Osta because I’m no longer a student and my new role is as a researcher. They said I can just submit everything to the Questura and renew the permit with the new contract.

However, I’ve always heard that changing from student → research/work usually requires a Nulla Osta, so this confused me.

Has anyone gone through a similar situation? Is it normal for this transition to be handled directly by Questura without a Nulla Osta?