Quote Originally Posted by BasicECommerce
You should brace yourself for a wild ride for the first week after changing. This will be when the search engines are re-evaluating your site. Sometimes you'll have excellent positioning and other times you'll be hard to find. After a couple days, things should begin to level out back to your baseline levels.

-BEC
Agree completely with this, its all a bit of guess work but if you already have good se positions then it shouldn't impact your new website too much.

You will also need to include an automatic 404 error message (page not found) redirect for those pages from your old shop that are no longer a part of your new one (different naming conventions etc)