From my first link
''What makes this possible is the fact that every smart meter being deployed in the UK includes a relay which can disconnect the household from the electricity supply. This is controlled by the utility and makes life easier for them by allowing then to manage connections and disconnections from a computer terminal rather than having to send an engineer out to gain access to the house. It’s part of the savings that they claim justifies the deployment of smart meters. It also makes it easier for them to switch customers to prepay if they are in arrears. Again, they don’t need to send out an installer to change the meter – they do it from their computer keyboard''
Note the words ''It also makes it easier for them to switch customers to prepay if they are in arrears'' So YES, they can switch you to another tariff without your permission and without notifying you.
They can also remote disconnect you completely but the rule says they must give warnings first.
All this is unlikely I agree , but the point is Smart meters make it possible when it is impossible with a standard meter. That is why the companies want us all to have a smart meter.