Conrad's Fate by Diana Wynne Jones

Someone at Stallery Mansion is changing the world. At first, only small details, but the changes get bigger and bigger. It's up to Conrad, a twelve-year-old with terrible karma who's just joined the mansion's staff, to find out who is behind it.

But he's not the only one snooping around. His fellow servant-in-training, Christopher Chant, is charming, confident, and from another world, with a mission of his own -- rescuing his friend, lost in an alternate Stallery Mansion. Can they save the day before Conrad's awful fate catches up with them?