Many people believe that SEO is something that you add to the site once it is ready. And yes, there are many cases when SEO is implemented after you’ve started, but if you’re about to switch platforms, then it is a really good idea to bring your search engine optimizer on board.
There have been many occasions during my career when I have been contacted by someone who needs help fixing the search engine optimization on a site that has lost all or some of its organic traffic when replaced by a new one. To me it is obvious that it is better to contact the search engine optimizer before you rebuild, rather than after you have lost your positions.
It is not that difficult, from an SEO perspective, to lead a migration even if there are many aspects to keep track of. However, you need to have solid experience, and a good foundation of SEO knowledge. By setting the requirements and strategy right as you migrate to a new platform, it is possible to gain positions rather than lose them. Losing is pretty common and it is not always easy to find out the root causes (it sometimes takes a very long time until the results of the work are visible).
Talk to someone about search engine optimization before you build a new site. There is much to gain from it, since all major changes pose risk to SEO.