You can change the behavior of a Custom Field. If behavior is controlled by workflow, the custom field can be made read only ...

You can change the behavior of a Custom Field.  If behavior is controlled by workflow, the custom field can be made read only or required at the workflow stage level.  When editing this field in Project Web App, a value must be provided before checking in or advancing the project to the next workflow stage.  Because required fields can increase work for users, carefully consider whether to make this a required field. Setting a Workflow Controlled Custom Field to non-Workflow Controlled while in Backwards Compatiblity Mode cannot be reverted.