The Tron mainnet has been up and running since May 31st and the TRX token migration will occur from June 21st to Jun 25 on what Tron refers to as the Independence day.
All TRX tokens held in a wallet must be deposited to a wallet on a supported exchange for the migration
In a tweet Justin Sun has confirmed these supported exchanges as :
Happy Indepence Day #TRON-sters in advance