Technically, yes... b/c Google averages in everything including page layouts with all the associated tags, etc. But in the grand scheme of things, I don't think it will hurt you that much if all the content remains. Just make sure that you continue to keep the site active within SE guidelines. Great question. I'm not an expert on the matter. This is just my opinion. I'm interested to hear others.
Sort of... If you change the page structure, it'll obviously affects your ranking for big G should have to recapture all your pages. However, there will be no difference if only stylesheets are modified.
Do you mean change the look of the site, or change the way it is structured? You can completely reskin a site using new images and new CSS without changing a single line of your html. Since Googlebot and friends don't to the best of our knowledge care about CSS, this is fairly safe.
I've never had problems with google, but I did have one my site that ranked #1 with msn slip past page ten when I changed the template. The change was so dramatic that there may have been other factors involved and it's happen to me only once.