Wordpress is not a magical fix for SEO. It all depends on how the designer handles the background coding and the different file system. The switch will show as no PageRank in the Google Toolbar (if you look at that) for a while.
I have pretty good page rank...
Don't fix what ain't broken.
That being said, there are some background coding things you could do with what you have that when added up will improve the SEO of what you have.