Remind me never to take legal advice off Yahoo !
Tupe doesn't last for 12 months - It's indefinite. Essentially the new company takes on the old company's T&Cs as part of the employee's contract of employment (excluding a couple of things around pensions etc).
Yahoo is correct that they would be within their rights at any time to make everyone redundant and have them reapply for their jobs with a 50% pay cut however they would have to conclusively prove that that was nothing to do with the switchover. In essence the onus would be on DFS to prove that at the time of signing the contract they could not have predicted the need to do this. If they could not prove that any employee affected would have the right of legal redress.
Under Tupe DFS have (almost) exactly the same rights to change contracts that NATS do now. And the same potential ramifications if they do.