Both domestic and international ticket holders can rebook for a flight before February 28, 2021—tickets can only be changed once.

Photo: Turkish Airlines
According to a statement on its website, the airline said: “In accordance with the decisions taken by the authorities of the Republic of Turkey and to protect the public health from novel coronavirus (COVID-19) pandemic … all our international flights are suspended until May 20, 2020.”
Previously, Turkish Airlines said it would halt almost all international flights until April 17, but has now extended the cancellation until mid-May. Meanwhile, domestic flights have been suspended until May 1.
Passengers who have purchased international tickets on or before March 20 for travel booked until the end of the year will have all change fees waived.
As for domestic flights, change fees are waived for tickets purchased on or before March 19 for travel booked until the end of the year.
Meanwhile, both domestic and international ticket holders can rebook for a flight before February 28, 2021—tickets can only be changed once. Rebooked international flight tickets must be to the same region as the original destination. Travelers can also choose to have an open ticket without a fixed return date.
More information here.