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I just read that if you get a site established in Google with one WordPress framework, lets say "Thesis", and then change to another one, lets say "Genesis", that you could lose all your traffic from Google (if you have any).
Is there any truth to this?
It depends a lot on the design. If you switch to a very complex theme with a bad loading speed and a lot of ads, it might affect you. But if the new theme is simpler, loads faster and overall better for the user, you might even see improvements.
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.
No it wouldn't affect how Google see your site. Google is more interested in your site's content and not the framework or cogs that run it.
If Google rescans your site and it can't find something that was previously there THEN it might re-rank your site for specific keywords or topics. Just make sure your site is showing Google all of your content. They pay smart people a lot of money to work on site mapping and with a product like WordPress I would imagine they have it and its guts figured out.
