Rumour is all there is on this topic so far, and most of it is as you say bruited about by scaremongers. The two options available that I've heard are equally unlikely:
1. Banks seek to exploit some obscure clause allowing them to vary the margin they charge on the ECB rate.
2. Banks seek to apply a special tracker account charge, to much the same effect.
I think (1) would have happened by now to anyone unlucky enough to have such a thing. The banks have not been behind the door to date on closing down any trackers by whatever means they have once they sniff a vulnerability.
It seems the (2) would require Regulator approval, and he's not in the mood for it. Even then, it could well fall foul of a court challenge as a unilateral change of a contract term if account management fees are not mentioned in the mortgage T&Cs.