I don't think Malfoy would be happy (at ALL) about changing houses, but I really don't think anyone would be asking his permission... I mean, since I think they're seperating that group to keep a Voldemort-uprising thing from happening.
But have we ever heard of anyone changing house before, i doubt it would happen, becuase malfoy wasn't on the line like Harry. The hat hardly needed to be on his head before it placed him
that was my thought at first, but I don't think they've
needed to have someone change houses before... and I don't think it'll be in any relation to the sorting hat, really... I just think that, because of the Voldemort coming back thing, Slytherin (or possibly just the little Malfoy-Crabbe-Goyle-Nott band) will be separated to keep them from getting together and trying to have a Dark arts uprising... so Dumbledore realizes Voldy's back, he sees that the children of the arrested death eaters are spending a little too much "quiet time" together, so he decides for the safety of all, to separate them.... the only way to effectively separate them is to put them in different houses... So he goes to them and doesn't say "Would you like to go to Gryffindor?"