I am going to change my design and layout, some of the text will change as well. How much will it impact my SEO rankings?
Will you be changing your urls & site structure? If so, make sure you setup 301 redirects. Once you upload your new site design, you will likely temporarily lose your search engine rankings for a little while. Basically Google wants to be sure that your site is still relevant and is not something they need to block. In time your rankings should come back. It is tough to say how your rankings will be affected after this temporary loss of position. If you do a better job of on-page seo, your rankings will likely go up a bit. If it is essentially the same, the rankings might not change much at all.
If your text is changed drastically, you may losing your SERP on several keywords. And i ever heard that changing from 3 column into 2 column and otherwise can cause your ranking drop temporary.