Best Licensing Option for Project

Hello Meline77 ,

Based on your message it appears that you are looking at on-premises? Have you consider Project/ Project Online or Project for the web in the Microsoft 365 cloud? Service descriptions here:
https://learn.microsoft.com/en-us/office365/servicedescriptions/project-online-service-description/project-online-service-description 

For the perpetual on-premises licenses, I think you'd either have to true up with with Microsoft when swapping licenses or ensure Project Pro was uninstalled if the user was not licensed etc. but this will probably depend on the license agreement your customer has with Microsoft. For users accessing PWA on-premises, they will need a PWA CAL but Project Professional includes a PWA CAL. Some details here for on-premises:
https://www.microsoft.com/en-gb/microsoft-365/project/enterprise-project-server 

https://www.microsoft.com/en-gb/microsoft-365/project/compare-microsoft-project-management-software?activetab=tabs:primaryr2 
For SQL Server CALs - this will depend in the SQL Server edition and license model: https://www.microsoft.com/en-us/licensing/product-licensing/sql-server 

I'd recommend you speak with your Microsoft licensing expert / software provider to clarify all license requirements.

Paul