It depends on your LEC, but I would expect your Yahoo order to fail / be rejected when they figure out that there's still service through another provider active. At least in Verizon territory, the first service has to be terminated and then you have to wait at least two weeks for behind-the-scenes tear-downs before your line is freed up to activate new service. Yes, they insist on disconnecting and undoing every last bit of the physical and logical service before turning around and reconnecting and redoing it all. They can't just switch the ISP over, last I checked.