What about the same day?
If you elect to 'upgrade with cert' during OLCI, depending on availability the system may give you the opportunity to standby for earlier flight (or perhaps to confirm for another flight, I can't recall).
It may do the same for LMU - I've never tried it.
You go to the airport and when the GA calls you up to exchange your old boarding pass for the new one, you say you've decided not to use the cert. Are you downgraded to Y on the earlier flight or downgraded to Y on the original later flight? (You check ahead of time to be sure that neither flight is oversold in Y)
[of course, maybe you never had the cert in the first place...]