Cloning a BD/DVD Title

[wm58]

This should be straight-forward, but it still confuses me, and I haven't found an answer in the forum (where is the users guide, anyway?).

Scenario:
Movie 'ABC' is in the database
There exists a BD version and a DVD version in the database of movie 'ABC'.
I want to add another edition of one of the above discs.

How do I open a 'clone' page, change the UPC code, etc., and save it?

It seems no matter how I do it, I get some kind of error message, or, the page that opens contains only an abbreviated short list of the disc title.

Ideas?