In the past here in this forum, it has been stressed several times that the definition of SDC is to be done at the airport only, however, the fare rules may or may not allow a "true" free SDC, meaning a fee would be needed to be paid in order to confirm, at the airport, to an earlier flight. Although this, with all intents and purposes is technically the same day, others have stated that this is not a true SDC but rather a change fee, which is typically more expensive if done before getting to the airport.

So I've always looked at the definition of SDC as being free as it usually comes with an unrestrictive fare class, and that restricted fare classes do allow for same day changes at the airport, but for a change fee (which has been explained here as not a true SDC per se despite the self-describing term). Edit: of course, just realizing that this probably is one of the perks limited to SE100K's as I explained above.

And with your scenario needing Z, this is my understanding (that might require clarification from others): this tells me it's a change fee and not a true SDC (which as I explained above is a free thing). My understanding is that a SDC puts you into any bucket in the same cabin with positive space and you don't need the exact fare class. If this were the case, true SDCing would be really difficult, IMHO.