Hi Chandana
There are a few major things to consider when updating the site:
**Keep same URLs **
Maybe the single most important factor is to not change allready ranking URLs. I do understand that changing the look of the site might require this, but you should at least do your absolute best not to change them. Also if you have a mutlilevel structure such as www.example.com/category/page be very careful changing the category since that will mean you also change all subpages.
I would check pagerank and incoming traffic from search for each of the pages, then make a note on what pages are most important. You can use www.opensiteexplorer.org and Google Analytics for this.
301 redirects
If you have to change your URL structure then make sure you do proper 301 redirects to the new URLs. This will not prevent you from loosing ranking on the page but will help you recover faster.
On-page SEO
Make sure your new site is optimized as far as possible for on-page SEO. Before you move, test the old page, then make sure the new page is atleast as good. Maybe also a good idea to look at headings and try to follow the same structure. Use the quite excellent tool from SEOMoz for this. You should find it here: http://www.seomoz.org/tools
Internal Link structure
This is not all that easy but try to look at how your internal link structure looks at the old site and move this to the new site. What footer links you got? What keywords you use in the links? What menus you use? If your new site is going for a dropdown menu this typically adds a massive amount of links to your site. This might be important.
Be careful
When making new sites its easy to get too busy designing and building the new site. Just play it cool and make sure not to make any mistakes such as leaving a no index tag or forgetting to remove a default robots.txt. Take an extra day to go through it all.
Spot mistakes as quick as you can
Make sure to use Google Webmastertools and Analytics to spot any problems and correct them as fast as you can.
Hope these simple tips helps you get going with your move.
Good luck and hope your new site turns out well.
Fredrik