How to update project class code on existing project with new class code?

We have an existing project with a class category on it. We want to change the code of the category, but it is with a brand new code. I entered the new code in Manage Project Class Categories task and made sure to back date its start date to before the project start date. The new code value does not appear in the drop down list when I try to edit the class category on the project. Is there another step or is this possible?