I'm not sure exactly what you're trying to accomplish.
In the Calendar API, there is an endpoint to Update a calendar event.
The first item there is calendar_event[context_code], which says it's "Context code of the course/group/user whose calendar this event should be added to." That makes it sound like it's the calendar something should belong to and by setting that value, you could change the calendar an event belongs to.
So, it sounds like if you do a Get a calendar event and do a Put with a different context_code, it should move it.
That's for a calendar event. For an assignment, it seems that it should always be associated with a course, so I'm not sure that it's changeable.
But you wrote that you looked in the API, so that's why I wrote I'm not quite sure what you're asking for. Maybe an example of what you're trying to do would help.
Disclaimer: I've not used the Calendar Events API, I'm just reading the docs, so I may be missing something there where it says one thing and doesn't actually do it.