I have a prepaid 512kbps tata wimax connection as well and I called them up a few days ago to get the 1mbps post paid connection but the customer care guy said thats not possible and I would have to get a new connection if I were to take a 1mbps post paid connection and get rid of the current one.
Worth or not... well really depends on what you do with your connection... if increase in speed gets you more productivity or saves time and makes more money for you then go for it.. all in all its for you to decide if that much speed is worth Rs2500 to you or not
I will get a post paid connection in Jan with a 1mbps speed as this month is already renewed with this prepaid thing.