Since we are all just guessing here (I am not privy to the data):
I suspect that demand does go up into and out of IST with the approach of the summer season probably above and beyond the capacity of an E170. When they say an A320 it could mean anything from a A319 to an A321, perhaps seeing all of those gauges depending on load, and I wouldn't be surprised to see an E190 substituted from time to time if the conditions are right.
Loads are typically pretty good on the E170 into and out of IST and I hope so are the yields. They can probably soak up any unused space on the larger aircraft through revenue management techniques by offering cheaper or even junk fares just to put bums on the seats, cover the marginal costs, and provide feed to the network. Of course, we are simply talking about the passenger cabin. The Airbuses will also offer additional cargo capacity which may be seeing more demand also.
But as I said, me just guessing.