Help me retain high ranking in search engine?
 

I have been running this website that does not have any AJAX, RSS or even blogs. I consider it really not growing and I only put updates in it a couple of times in a month. The HTML coding is done manually. Lately, it reached a high rank for various keywords and got optimized. It topped Google's 1lac, and 25 of 27lac. I have been thinking if this MS (Joomla) could be helpful to my plan of renovating my site. I want some changes to its navigation, structure and sitemap. But I have the worry that it might turn out into a new design and eventually will lead the ranking to fall. Am I right with my anticipations? Do you think Google's high ranking today will be affected by revamping? Please tell me what to do to maintain it.