You should have the ability to change the type on WTDocuments, EPMDocuments and WTParts from:
root hard-type to a sub-type
sub-type to root hard-type
sub-type to different sub-type (of the root hard-type)
For example, WTParts may be created initially using the "Part" hard-type. Later, the business determins that a new sub-type of WTPart (i.e. Electrical Part) is needed and the new sub-type is created. Some of the existing "Parts" need to be converted to "Electrical Parts". The UI should allow such a change. The user should have the option to apply the change to all iterations or limit the change to the latest iteration. Common attributes between the old and new type would be retained, non-common attributes would be lost (on the changed iterations only).