Changing a custom attribute type within a project area

Under the Process Configuration tab of a process template, I can simply edit a custom attribute to change its Name, ID, and Type. However under the Process Configuration tab of a project area, editing a custom attribute only allows me to change the attribute's Name.

How can I change the attribute's type in the project area? I'm merging a process template update into an existing project area by updating the project area's process configuration XML. This didn't result in any errors for the changed attributes. However, I get errors on presentations for the changed attributes. For example, when I changed a custom attribute's type from Small String to Timestamp, I get the following error on the attribute's presentation:

Kind 'Timestamp' does not support attribute 'createDate' of type 'smallString'. Supported attribute types are: timestamp.

When I view the CreateDate attribute in the Project Configuration tab, I see its type as Timestamp. However, the presentation error message implies that the CreateDate attribute's type is still the old type of Small String; that is, the createDate attribute's type is not really changed to Timestamp even though no error is given on the Process Configuration -> Work Items -> Type and Attributes -> Custom Attributes.

Thanks,
Geoff Alexander