If I change my home page content will I lose my rankings? I'm thinking about redoing my whole site to improve it but I really would like to keep my rankings. can I?
It depends. If you have a high ranking for "blue widgets" on your blue widget homepage and you change the main topic of that homepage to soccer then yes, you'll lose your high ranking for "blue widgets". Basically, Google will look at your site the next time the spider comes around and if it's the same topic but with improved and fresher content there's a good chance you'll keep a high ranking.
Changing the design will have a major impact on Page Rank. The old page has PR and the new one does not. It will create "broken links" that also will create a PR loss. Changing from one platform (.NET) to (PHP) will create a completely new site that has to be indexed from scratch. And finally, by changing the "content", you will change the existing SEO quality that will alter the SERP performance. Cosmetic changes will not have an impact but text changes will. One important concept you should use is a 301 Redirect on a page by page level. Google will then pass the PR to the new page (so I'm told) and it will give the new page time to get indexed and PRed. It might be better to run two sites in parallel.