You can either Sweep (which will create an onchain transaction that sends the BCH from the old "btc" addresses to new BCH addresses in your BCH wallet)... or you can Import the keys into a BCH wallet (use the "Import Bitcoin addresses or private keys" option when creating the wallet in ElectronCash).

It depends what you want to do... if you want to HODL your BCH, then Sweep is the better option as you're moving the coins from the "exposed" keys. If you're just wanting to dump the BCH... importing the keys and then just sending them to an Exchange is probably "cheaper" and faster (saves paying and waiting for the sweep transaction)