By default, Siebel out-of-the-box does not allow you to change the Primary Flag in MVG applets. This includes Clinical Site, Accounts, Protocols, etc.
Here is an example:
On the Site Management screen, press the icon for the Team MVG:
This opens the Pick Team MVG Applet. Note that the Primary Flag field is read-only.
Now, in Siebel Tools, for the relevant business component, add this user property:
MVG Set Primary Restricted: Position with the value of FALSE. Note that, in this instance, Position is the name of the multi-valued link on the Clinical Protocol Site business component.
Now, after compiling this business component, you are able to change the primary user: