Can we change Manage Project Class Categories/One class code per project after initial set up?

Summary:

Setup and Maintenance > Project Financial Management > Project Foundation > Manage Project Class Categories :

Class Code "Department" was originally set up with "One class code per project" not checked. We would like to restrict this option so users can only select one department on a project but the option is now greyed out so we cannot change it. Some of the other set ups allow the options in this section to be changed so not sure why some are able to be changed but not others.

Content (please ensure you mask any confidential information):

We would like to restrict Manage Project Class Categories/One class code per project so only one Department can be identified on a project