It depends on what is changing. If your old pages had different URLs (which is probably the case - they were probably .html files) then it would be very helpful to do 301 redirects. This will tell search engines that each old page, which no longer exists, has been relocated to a new address.
As Snakeair says, you will probably see some changes in ranking but it should settle - if most of the site has the same content, you should stay around the same rankings - as long as you do the redirects. I would try to make sure to set up the basics like WordPress permalinks, and an SEO plugin that lets you set a unique title tag and meta description for each page/post/etc., and verify they are all to your liking before you go live with the changes. If you go live with permalinks you don't like and then later change them, that will lose more SEO juice.
Elaine, a.k.a. RedScooter
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