Whenever you change a theme, be it the entire framework or even just a child theme within the same framework you run a risk of affecting your Google ranking. However, the risk is not just about losing rank...a theme switch can affect you in the positive as well as the negative.
Switching from a poorly-coded, non-optimized, non-microformatted theme to a well-coded, faster, structured-data intensive theme can markedly improve your site's positioning in Google. Generally speaking, the frameworks are not your biggest concern. At the upper end of the framework spectrum (where both Genesis and Thesis sit) there is probably little real difference in how they handle general SEO elements.
What really varies in quality are the child themes for the various frameworks. I've seen some real stinkers coded for Genesis, Thesis, and almost everything in between. Before you switch to a new theme, child theme, or framework take the extra time to create a test site that is visible to Google (use dummy or sample data) or do A/B split testing and really spend significant time with GWT and/or your SEO tool of choice to get a sense of loading times, structural deficiencies, and any other advantage or problem you can measure.
As mentioned in other answers, the one big thing that will drop your ranking every time is a change in permalink structure. Changing themes alone will not rewrite basic post and page permalinks. What you need to be careful of are custom post types that are generated by the theme's
functions.php file or called as an include from
functions.php. When migrating to a new theme, make sure any custom post types are fully accounted for prior to making a complete switch and this is a case to be extra-careful when switching frameworks. CPTs frequently require custom template files and those may be coded in a framework-specific manner, particularly if they need to make calls to postmeta to do whatever it is they need to do.