In the Modern UX, due to the auto save functionality, when new Cost Plans are created, it has become impossible to change the Grouping Attributes. As soon as the Name is entered, the Cost Plan gets saved with a set of default values, and cannot be changed thereafter.
Population of default values for "Grouping Attributes" attribute in the "Cost Plan" object.
Remove default values against the "Grouping Attributes" attribute: Administration -> Objects -> Cost Plan object -> Attributes tab -> Grouping Attributes attribute