The effects of changing a theme

Will changing my WordPress theme on a site that is over 6 months old and ranking fairly high effect affect the rank? All existing blogs etc. will remain.

Possibly for a few days. Had a site in a similar situation where I switched the theme due to some problems with it. I switched to a more cleanly coded them and after dropping a couple spots for a day or two it jumped to a better position than before the swap. So IMO yes it will affect it. For good or bad I suppose that depends upon the themes you are using.

The theme I am switching to will have partial parts of all my blog on it and link to the full blog, does this offer any benefit over a site that only displays a link the blog page?