Within 24 hours, you can do a same day confirmed change for free IF there is the same fare class available. I'm almost 100% sure you won't be able to find that fare availability, so you won't be able to do it in Y. If you buy up, then you'll be able to SDC to any flight that has open space in F, regardless of fare class, so you'll have a lot more flexibility. I don't believe you'll be able to SDC to PIT->LGA->DTW->EWR though (even if there is space), because that sounds like a broken fare that the website puts together for you.