I’m searching for the option for changing my keys… I don’t find it. “Changing your secret” do not seems to change the keys, so if some body can help ? 
not sure what “Changing your secret” is about
however, you can import existing keys at settings/advanced → manage keys.
password-protected keys need to get the password removed before import.
Is it possible to keep the old keys to decrypt the old messages ?
the old key should be kept, yes.