Dynamics CRM 2016 on-prem user extension

One of our customers, who is still on Dynamics CRM 2016 on-premise and does not want to migrate at this moment, wants to buy additional users. Customer has CRM 2016 Professional CALs. According to our information the additional users have to be licensed via Dynamics 365 Sales (Software as a Service) via Dual Use Rights and Downgrade Rights.

Question: Are Dynamics 365 Sales Professional licenses sufficient or must the customer purchase Dynamics 365 Sales Enterprise licenses for the additional users?