Consider the risk. Change all the urls and you could lose all that nice ranking. Leave well enough alone. Try a cart with a secondary url and see how well it works. If it works well for you then work it into your existing site.

You should be seeking a shopping cart solution that has a custom url rewrite function.

From what I am reading I think you should at least check out americommerce:

AmeriCommerce Knowledgebase

I do not currently utilize americommerce.com but I am considering them for a nest of sites. I currently utilize volusion and magento and I sware by volusion and my magento experience has not been kind (please don't go there!)

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