It's very easy to maintain your search engine footprint when you are converting an old site over to concrete5. The first thing you will probably want to do is get rid of the index.php that appears in concrete5 URLs by default. To do that, you need to enable pretty URLs in System & Settings > SEO & Statistics > Pretty URLs. Tick the box to enable pretty URLs and paste the code it gives you into your .htaccess file in your concrete root directory. That will remove the index.php from your URLs.
Next we need to add pages for each of your site's existent pages. Let's say your original site had a page at mysite.com/mypage.php. You want to keep that same foot print so the search engines don't notice a change and throw away all your built up page credibility for that URL, even though your new page would have the same content, just a slightly different URL. But we can make sure that the change-over is seamless.
To create that same path for your newly created page within concrete5, you would create the page, and then go to page properties.
Here you can enter an additional path that also includes the file extension (in this case, .php) extension of the actual file. You can include it in the canonical URL or in the additional paths field or both. Either way you will be able to recreate that URL perfectly so that your link juice flow is uninterrupted! Additional paths use a 301 (permanent) redirect, so that also keeps the search engines happy.