My experience -
After combining the accounts (to the Marriott account):
1. Cancelling Marriott reservation - points immediately back to my account
2. Cancelling new SPG reservation (booked after combining the account as test) - points immediately back to my account
3. Cancelling old SPG reservation - points did not come to my account - short of 375,000 points. No real time frame or answer to when it will be resolved