It may possible that SERP must be affected but after sometime you can regain your SERP. If you want to know about more " How to boost your Blog Traffic " read goolgegenie blog.
Serps souldn't be affected (but it can). If the template doesn't make major changes in you layout, if is SEO Friendly, if... there are many aspects that you have to take into account. You can do a backup and install it on a different server to see if the content is affected. But I suggest this: do a manual redesign on your template removing just the images and changing the css styles. Be inspired by your new template in this redesign. This should be 100% clear and not affect any SERPS.
It can effect if the theme has lot of flash element and invalid markup, also make sure to check the permalink structure that may change with few theme.