I have about 5 blogs that are set up as static sites and I am nowing doing the SEO work. If these blogs start getting trafic I want to invest money and have a bespoke site. How easy is it to change the template will this affect rankings if content stays the same.
If you are asking about changing a blog theme then that is a simple task - it will involve you uploading a new one via ftp or with the new WP version you can do it through the admin panel. And changing a theme be it a static or blog should not effect you PR at all. But if you asking about changing a static site into a blog, then changing the template alone wont do it - this will involve a some work on your part or a third party if you pay to input your static site content into the new blog. There is a php script I read about sometime ago that can do the convertion, but for the life of me I can not remember where - sorry. Maybe a google search may bring it up. Good luck.
As long as your new template doesn't dramatically alter the text content or code, then you should be fine. I recently changed the theme on one of my highest ranking WP sites, and used a duplicate content checker (in reverse) to try and make the new theme's code as similar to the old one.