I should be able to change an ExactlyOne Property to Optional

(Missing Feature, To be Fixed , Priority: Medium, Test Status: No automated tests yet , Reported By Justin du Coeur, Effort: Moderate)

Data Model, Properties

Summary: It is not unusual to create an ExactlyOne Property, fill in a bunch of data, and only then realize you don't need to fill that Property every time. So it would be helpful to be able to change it to Optional.

This is not a minor change -- to make it work, I'll have to enhance the serialization format. But I'm beginning to think I need to do that anyway, so this may be what motivates it.