Merged my Marriott & SPG accounts into Marriott -- worked. Merged my wife's SPG & Marriott accounts in her new Marriott (former SPG) account -- also worked. When prompted to enter the other program's credentials I used the Marriott Rewards number (either the legacy one, or the new one assigned to match a legacy SPG one) and the password that went with THAT account. Using email/username I had problems, too.