But surely it wouldn't take her to her last year? The Marauders managed it in their fifth year, starting in either the first or second - second, I'd assume, as it takes them a while to work out why Moony buggers off once a month - and they changed okay.
Admittedly, the legal beginnings of turning into an Animagus and the arduous training can only start at N.E.W.T level training, but just as the Marauders (And I'd assume Rita Skeeter) found the information for it in the school library restricted section, surely Roze could do it to?
I mean, she will be getting a fairly oblivious, air-headed DADA teacher in her second year...