I want to dump some of my Bcash coins stored in my paper wallet using trezor sweep function. I don't want to move the bitcoins stored on the same address. What risks am I taking? Only replay attacks?
It should be easy to do without risks. Import your BTC to Electrum wallet and from there, import the private keys to Electron Cash a fork of Electrum wallet and then send to an exchange and dump all you want. After you have done this send your bitcoins back to your hardware wallet. I do not see any risk if done such way but I am not that experienced with BCC so proceed with caution. Here is a link to start to import your Trezor private keys to Electrum.
https://doc.satoshilabs.com/trezor-apps/electrum.html