Ensuring optimized content is properly transferred to a new website is just one of the reasons you want to get an SEO involved in the development stage of a new site. Web designers are typically just focused on the new site and don’t think about what is necessary to maintain previous optimization efforts. In most cases, the new designers will transfer over raw content, but what may be missing is the HTML markup of that content, along with title tags, meta descriptions, alt tags, etc., that should accompany it.
If you’ve spent any amount of time optimizing your website, specifically content and meta tags, you need that information to transfer, intact, to your new site. The only exception to this rule is if you’re generating all new content for the new site. Typically, that won’t be the case. You’ll write some new content for new pages, but rarely will all the old content need to be discarded.
As a general rule, it’s a good idea to change as little content as possible between the old site and the new site. Many things can go wrong with the development of a new website. If your content stays the same and you see rankings or conversions come crashing down, you have already narrowed down the culprit. It’s something with the site itself. However, if you redo the content throughout the site, you won’t know if the problem is the coding or the content. And it can take months to narrow it down.
Keeping the content the same from the old site to the new allows you to rule out any possibility that the content is the issue. That alone can save you a lot of time having to re-optimize. Instead you can start looking to site architecture to find and fix whatever caused the problem.
If you do feel that the new site requires new content, I recommend doing it slowly over a period of time after the new site rolls out. That way you can track the metrics to see first how the new site does and then how the new content performs. This will allow for quicker changes should something go wrong.