You have to create new wallet (that will be backed up with Shamir backup) and transfer your funds there from your previous wallet by doing regular transaction.
You can use only one wallet (represented by one seed - either standard or Shamir) at the time on top of that you can create almost unlimited amount of hidden wallets.
So practically If I want to stop using my simple wallet and move to Shamir I’ll need to (And assuming I have only one Trezor device):
Create an advanced (Shamir) wallet and copy the public address for the funds to be moved. This will prevent me from accessing my simple wallet easily since now my Trezor is represented by the advanced seed.
Sweep to my simple wallet, and send funds to the mentioned public address.
Sweep to my advanced wallet and start using it.
Is it correct?
Also, what about the device password (the digits)? is it shared between all wallets or a new one is needed when a new wallet is created?
If the latter is true, how can I access the simple wallet after creating the advanced one?
Also, you set your PIN every time you either create or recover your wallet (step number three on the picture) so it is not being shared between the wallets. (but you can of course use the same one when setting it up).