Using one process (Call) to generate a new one (I/R) isn't too difficult, you're essentially copying over values. We do something similar converting Incident-->Request for those that get created incorrectly via email. The tripping-point is moving over all the objects, i.e. Attachments.
When i get some time later, I'll copy in examples of what we've done.
Don't suppose you have had chance to find some examples have you?
Has anyone come across this that could help please?