new site - concerns about losing seo benefit of old PHP site

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Company has a site that is old and developed in PHP and not overly search engine friendly - having said all that over 60 of the pages index really well in the search engines.

Our MD is obsessed with developing a new site and has brought in a new design agency to do the work - they are currently blasting away with the new site. I'm really worried that we are going to lose all the existing SEO benefits of our current site - what is the best way of keeping these?

Any advise would be greatly appreciated