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Trezor isn’t a magic box that you plug into a wallet for “additional protection”.
Trezor is a _wallet_ with **its own address**. When you add it to Metamask, you will have two accounts, one on Trezor that is secured by Trezor, and another, the old one, on Metamask, that is still exactly as (in)secure as it was before.
(you _could_ import your Metamask seed into Trezor and get the same address from Trezor …. but that _doesn’t secure the Metamask account_. The Metamask account is still as much in Metamask as it was before and still exactly as (in)secure as it was before. Except now you have a _copy_ of the insecure account in Trezor too. Don’t do this.)