I have a website that is picked up easily by many search engines. However, I want to update it, completely change it and create new files with different names. Does this mean I will lose the search engine facility? Also if I change ISP and had a different web address would that ruin the good search hits that I get at the moment.
But to answer your question; no, changing your site (no matter how much) will not have an adverse effect on your Google Pagerank or standing with any other search engines. On the contrary, new content is advantageous to keeping that ranking in a huge and competitive market.
The most vital thing is to keep your domain name; if you want to change it for any reason, keep the original hosted and setup a redirect. For new pages, keep your metatags relevant and up to date.