we've thrashed around the topic today on the msmom mailing list today. turns out that copying an a rule does not preserve the content of the product knowledge tab.
other interesting thing to note is that the “update” feature of mp import does not retain the override criteria or threshold changes. the only thing is holds on to is disable/enable, company knowledge, and any rules you may have created for yourself.
the recommendation is still to copy any rules that you plan to modify and disable the original. as long as you're going to do that, you might as well move it into its own custom rule group so that you can export them at will and import at will w/out the fear of losing any of your work. i've been using sharepoint services to maintain a list of mom rules that i've modified over the course of my history with it.
oh, btw, you can copy the product knowledge to the company knowledge of a copied rule. not sure that it's the same effect... but at least you have something to reference. some of the tools for MP authors may allow some more indepth editing... who knows?