Even though Turkey is only a "Candidate" country of the EU, EU regulation 261/2004 still applies as you were departing from a member country (VIE, Austria).

For such involuntary downgrading of flights less than 1,500Km you entitled to minimum compensation of 30% of "the price of the ticket" (€450 if your ticket cost was €1500).

More here:As to how to claim this I am not sure. Perhaps contact the "National Enforcement Body" in Austria.