There is no question that we don't need them. The competition in the entrance exam is very fierce and it is increasing every year as the number of students appearing are increasing. Therefore increasing number of seats is very important. Thus we need more IIT's.
But converting the existing institutes into IIT's is not a good idea in my opinion. This is because just upgrading the institutes won't change the studying and work culture there which are very different for IIT's. Moreover this will not make any difference to number of engineering seats if we see as a whole. Thus we should upgrade the existing institutes and establish new institutes for IIT's.