Just curious to know that changing existing script to a new one and its effect on SERP. I want to change my existing script ..but I have 2/3 KW ranked in 1st page of google that send me pretty good hits. Will I loose my ranking? If yes how long it will take to recover my ranking and what measures should I take to recover it.
I don't think so, but you should keep your onpage factors same for new script too. You can also try building more backlinks.
Changing a script? Do you mean changing your web page(s)? If your referring to changing your web pages then yes it can have an impact on your current search results. This is due to changes of the on-page SEO, internal linking and content structure. Now, you can do it correctly and retain your current ranking if not improve it, but you need to follow the same structuring as you are currently. If you referring to an actual php\perl script then no it won't effect it unless it actually renders HTML.