I think you have a slight problem with timing. I think his change would have happened before the late 1930s/ 40s, when he may or may not have killed Ariana. I think that would have shown him that wizards are just as capable of violence, and stupidity as Muggles are and that would have had far more effect on him then the anti-semitism in Germany. I know this is quite a major point, but I think when dealing with Dumbledore you should never forget he holds himself at least partly, if not wholly, responsible for his sister's death, which, according to the lexicon happened in 1899 (though I have no idea how they work that out).

The interesting conflict you would have is why Dumbledore did not step in to stop Grindelwald in the 1930s rather than wait until 1945.

Having said that I think Dumbledore is quite a cold man. Snape points this out brilliantly in DH when (I don't have it with me)he is horrified that Dumbledore is sending Harry to his death. Dumbledore risks Harry's life throughout the novels (I mean, letting an eleven year old deal with saving the philosopher's stone?) with the sole purpose of making sure he becomes the man he needs to become to defeat Voldemort. Therefore if Dumbledore comes off as a bit horrible- idealistic and with high aims but few qualms about how those are achieved - I don't think that's OOC.