My profile is currently set up to earn Marriott points for my stays. I want to change this to US Airways shortly (for the Grand Slam) and then change it back to Marriott again. I have two Marriott-specific questions about the speed of this that I hope someone can help me with:

1) Let's say I have already changed my earning to US Airways. I check out of hotel A this morning having earned US miles. Can I call MR during the day to change back to Marriott points and then check into hotel B tonight and earn Marriott points? Is it that fast?

... separate scenario next...

2) Does anyone have any experience during a multi-night stay of having some nights credit to an airline program and then have the remainder credit to MR?

Thanks in advance.