Suppose we have a non-NIT institution X, and an NIT Y. Suppose Y is better than X. Lets say the quality of X is 5, Y is 7 and an IIT is 10. It is better to upgrade Y to an IIT than X because, even though one can put money to build the physical infrastructure of X to the level of an IIT, one can not get rid of the existing faculty of X. Even though one can hire additional IIT quality faculty for X, their will be an imbalance and tension in the overall faculty as the holdover ones will be of be of quality 5 while the new ones will be of quality 10. It would be much better to upgrade the NIT Y to an IIT as then the faculty mixture will be of quality 7 and 10, which is more stable and tension free than a faculty mixture of quality 5 and 10. Now if X happens to be in the same state as Y, so as to make sure that the state has an NIT, the institution X can be upgraded to an NIT. In that case X will have a faculty mixture of quality 5 and 7, which is more stable than having a faculty mix of 5 and 10.
In reality, there are additional difficulties, as just naming X as an IIT does not change the perception about X overnight. Thus better quality faculty would go to Y, when it is made an IIT, than to X, when it is an upgraded to an IIT. Thus it would be a much difficult uphill climb for X than it would be for Y. Similarly, making X an IIT will cause a bigger damage to IITs brandname than making Y to an IIT.
Continuing with our example, it will cost the same to upgrade X to an IIT, as it will cost to upgrade the NIT Y to an IIT plus X to an NIT. This can be seen by assuming that X has an expenditure of 5 crores, Y has an expenditure of 7 crores and an IIT has an expenditure of 10 crores. To upgrade X to an IIT one needs a new investment of 10-5 = 5 crores. To upgrade Y to an IIT one needs a new investment of 10-7 = 3 crores, and to upgrade X to an NIT one needs 7-5 = 2 crores, which adds up to 3+2=5 crores.
In summary, while it costs the same to upgrade X to an IIT versus upgrading X to an NIT and upgrading Y to an IIT, the later results in a better IIT, with a better integrated faculty, and with less damage to IIT brand name. Hence the government should consider NITs while making a ranking list of institutions for upgradation to an IIT, and for the NITs that are upgraded, the best non-NIT institution in that state should be upgraded to an NIT, so that as before every state has an NIT.