If I was the Moff, I'd retroactively add the Silence into the background of several scenes within Season 5 on the DVD release. Very subtly. You wouldn't even notice them if you weren't looking for them, and maybe you did notice on first viewing and you forgot... And maybe I'd put them in different scenes in the different region-coded sets, so the Aussies would spot them in different scenes to the Americans or the Brits. And then, as the most obsessive among us start to crow about how they'd been in the original version all along, those fans that never bought the DVDs but merely watched the show live or downloaded it would be so very, very confused. And even more scared than before.