Use the bip39 tool to derive the ETH addresses and private keys from your 12 word mnemonic seed, use the private key to login with MEW, transfer all tokens to Ledger.
Thanks for your suggestion. That's one of the cases that I need to keep in mind but I don't want yet. It's like a last resort thing for me.
How about letting my tokens stay in the address? Any idea? As much as possible, I don't want to move it.
As long as you have your 12 word seed in a safe place that should not be a problem... you can always install Metamask on another system and recover your funds.