I have a site hand-coded in CSS and HTML with some PHP touch and now i am converting it into wordpress to manage the site more easy. If i convert it into new platform like wordpress would it's ranking in google (SERPS) change? Would it fall and can i recover?
Yes it is possible on the initial phase you will see a fall or even ur links may disappear from the SERP's , but you will regain them Lately.
If you can retain these to be almost exact like the old one, then it would be a less chance for troubles. - Site Structure - URL Structure - Content - Meta Tags / H Tag / Bold Tag - Internal Linking Structure - Title tag on each individual page
as long as the code is clean and the content and structure remains the same you won't lose rankings, although it is unlikely that this will be the case if you move to wordpress