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You can use a passphrase with your existing seed, but that will generate a new wallet. The old one will still exist. If you wanted, you could transfer the funds from your old wallet (w/o passphrase) to the new one you would create. You just need to copy the address of the new wallet, unplug your trezor and interact with the old wallet. Essentially both wallets would be yours one with passphrase, one without it. Using a passphrase is definitely recommended as its an additional layer of security. If you use a passphrase write it down and make sure you don’t make typo errors. If you forget it or enter it wrongly initially without noticing, you could loose your funds.