a little change in content and a major change of design of a website ... will that result in a fall in page rank ?
I don't think a little change in design and content will affect your page rank.Rather changing contents a bit on regular basis is good for seo.
Not sure about PR, but its good for SERP. I have changed my website about 20 times in the last year, and I have maintained a PR3 on the homepage for 2 years.
PR is not related to design. It is only links which makes PR. But your design change give big impact for your website in terms of visitor's response and even in SERPs.
PR should only be related to Backlinks (but no one knows for sure how exactly Google works it out), but try keep the Content, Meta Keywords and Titles the same as well as your H1, H2 tags.