I should be able to change the Collection of a Property

(User Story, Investigate , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Editor
Summary: In particular, we need a straightforward way to change an ExactlyOne to Optional.
This needs to be fairly smart, along similar lines to changing a Property's Type -- not all changes are legal. But going from ExactlyOne to Optional is uncontroversial, and probably fairly easy.