jwolthuis - 9:14 am on Oct 18, 2011 (gmt 0)
It's not always possible to exactly map URLs from one eCommerce provider to another.
The OP's worry is that a new eCommerce provider may organize his catalog differently, possibly splitting/combining product variations, separating product reviews, and ultimately changing his basic URL structure.
Some providers include the parent-category in the URL; some don't. Others provide alternate organizational schemes that go beyond simple categories.
The OP is concerned that the SEO he built on his existing URL structure might be affected, if less than 100% of his existing URLs can be redirected exactly 1-to-1 to the URL structure of a new eCommerce provider.