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Hi If you're URL / permalink is not changing than in theory all the off-page SEO for the page will be unaffected since the page still exists. So external link juice still has some where to go (to coin a phrase). But beware of the anchor text of the link (well be aware) - more about this below: Andy makes a good point in that if you are changing manufacturer than most likely some of the text will change ( obviously "Nike" changes to "Gap" for example). So from an on-page SEO perspective the page has changed and this might affect how well the particular page ranks for "Nike Green Sweater" since quite obviously that particular page is no longer about Nike Green Sweaters and assuming the search engine bot is doing its job it will work this out and adjust indexing for that brand specific keyword combination. To deal with that potential problem, I would suggest adding a link to the Nike Green Sweater page on the Gap Green Sweater page - you can do this quite naturally by adding a "related products" link or "people who bought this also bought" or "looking for Nike Green Sweater?" (for example). This passes some of the link juice for your page to the Nike Green Sweater page. Helping Nike Green Sweater Wearers...or Another Reason for an Internal Link: Creating an internal link to the Nike Green Sweater page also helps people who came to your site through a link that explicitly referred to "Nike Green Sweaters" and were looking for "Nike Green Sweaters". (You could use a tool like AHREFs or Google Webmaster or Open Site Explorer to find the anchor text being used by people externally linking to you to get a sense of the scale of this "problem". )

If you are sold on prestashop and are sure you have the URLs taken care of then that is pretty much all you can do in that regard. I still think you should look deeply at the links after the site is done, and possible do all of this on a dev server to make sure everything copies over as well as you hope. I am not super familiar with Prestashop. So I am hesitant to give much specific advice on it. Some of my concerns come more with that. I personally use Magento if the client is sold on a eCommerce based CMS. They are the largest, most SEO friendly, and the most protected when it comes to plugins etc. Plus eBay owns it now, so you have that behind it. Regardless, I don't know if I really even have '10 things" for you. Check links before and after to make sure everything matches up. TEST, TEST, TEST. Try to break your website, on a dev server. Someone else will if you don't, that way you can identify it before somebody else does. Check plugins. Plugins are one of the best ways to ruin a website. One update, or lack of an update can throw an entire website offline. Accept that any change you make will have an effect. SEO is just like Newtons laws. Every action has an equal and opposite reaction. Now, in the long run, switching to a CMS could be extremely beneficial, at first it very well may hurt you. I would highly doubt that it would drastically impact your rankings, but in my opinion, you cannot make a change in this world and not have there be some sort of ripple felt somewhere else. Good Luck