I've just encrypted my drives using Bitlocker but since I don't have TPM, I went with USB key. Is it possible to switch that method to PIN or is it too late now and I have to re-encrypt the drives?
Well, apparently it's not possible.
Only four options are available:
TPM only TPM and PIN TPM and Startup Key USB only
Therefore, USB it is for me ;)
It is possible to change the protectors, but if you don't have TPM then only USB is available to you.
I have TPM and use TPM + PIN + USB as the protector.
The most important thing is for you to store your recovery key somewhere safe as without that you will NOT be able to access your data if you lose your key.