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?
Moving BCH first was replay protected from the start. Moving BTC first was initially only opt-in replay protected, but the Bitcoin ABC team made some last minute changes before the fork that should replay protect BTC transactions by default as well (assuming the majority of BCH nodes and miners have updated their client software accordingly).
So in theory you
should be safe from replay attacks.
Apart from that, remember that by sweeping the paper wallet you're exposing it's private key to your system, so the regular caveats about online hot wallets apply. Emptying your old paper wallet and sending your BTC to a new cold storage address (eg. your Trezor, since you mentioned it) would be prudent.