this is an interesting article I just found here, saying the 5 top things to avoid changing for keeping your current SEO rankings are:
1) changing to a new domain name
2) altering your URL structure
3) significant changes in content
4) changes to whois records
5) changing the overall site content theme or niche

of course, the article fails to mention that if you do change things for the better, you may actually get better SEO too :crazyeyes: