Why the Portfolio drop-down field is inactive on a project details page? How can I move a project to a different portfolio?

General Questions
Q: Why the Portfolio drop-down menu is inactive on a project details page? How can I activate it?



A: Current user should have the "Project - Edit" permission enabled in the project:




Once the above requirement is fulfilled the field becomes active:



NOTE. In case if the above permission is enabled but some portfolios are not shown in the drop-down list then current user is not included to such portfolios. 
The solution is to include the user to the portfolios or enable the "Manage all portfolios" permission in the user's global role.