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Theoretically speaking, a Domain MozRank jump from 5 to 9 in two weeks if enough new links were discovered. That is the only factor involved. MozRank and similar metrics are updated about once per month when the Linkscape crawl is completed and the data is processed. The figures will be stable for about 29 days each month then one day the new data is loaded and you will see the resulting change. A site moving from mR of 5 to 6 is a very positive change. For the same site to move from 5 to 7 is a huge jump and would require an exceptional amount of link building effort. Moving from 5 to 9 is not realistically possible. The likely causes are: an error on your part. The site's mR was either not previously 5 or is not presently 9. an error with Linkscape where it either did not see all the links before, or is improperly crediting links presently. an error with whatever tool you are using to see mR such as the MozBar. Maybe the page did not load properly and you were seeing the mR from the prior page. This is the most likely issue in my opinion. There are very few sites with mR of 9. Can you share the URL? If none of the above issues describe the situation I would recommend opening a ticket with the help desk (help@seomoz.org) and provide the details.

Mystripping the category is "safer", because I've seen so many eCommerce that assign a product to more then 1 category. This causes at least 2 urls with the same content... Duplication with all the risks dupes have for a site health.

Agreed. The risk of losing juice from a redirect would keep me from moving forward. The only way that I would consider redirecting the old page is if the new page provides better and more relevant/current content. If you don't plan on improving the content and are only using duplicate content then there is no need to change the page or URL.