Once you authorize the Flexa Capacity dApp to migrate your Flexacoin tokens to Amp, it will execute a single transaction that:
Sends your FXC to the Ethereum burn address (at 0x00···dead), and
Mints new AMP from the Ethereum zero address (see an example).
By sending your FXC to the Ethereum burn address, the dApp ensures that these tokens cannot be used for any other purpose going forward.
Although these tokens are technically not "destroyed" (and will still count toward Flexacoin's fully diluted supply of 100 billion tokens on blockchain scanners like Etherscan), they are virtually impossible to retrieve and should be considered completely worthless.