Retaining web-ranking for our website while changing URL

We have a website called iVolunteer.org.in which is fairly high ranked on various searche engines. For eg, a google search on 'volunteer bangalore' will desplay it as No.1 Same for 'volunteer delhi/mumbai/chennai' and in top 10 for 'volunteer india.

Now we have developed a new version of the website. However we want to change the URL to iVolunteer.in Presently both websites exist parallely.

My issue is that how do I retain the high rank and history of the old website on the new one since all previous references are to the old website.

Help ! iVolunteer brings skills, expertise and passion to the social development sector in India.