What would happen to an event in the past, if time itself ceases to exist? Without time, has what existed (such as the writing of this sentence) never happened?
A total disappearance of space-time seems impossible, because it implies the non-existence of something undeniable: that something that happened, happened. We could deduce either that space-time cannot cease to exist or that it is not the real theatre of the events.
Saint Thomas asked himself a similar question (Quaestio quodlibetalis XII), when he asked "utrum Deus possit reparare virginis ruinam", namely whether God can change the fact that a virgin has lost her virginity. Thomas' answer was clear: if the question concerns spiritual matters, God can certainly make reparation for the sin committed and give back to the sinner the state of grace; if it concerns physical matters, God can restore the physical integrity of the girl; but if the question is logical and cosmological, well, not even God can make that what was once done, was not done.
Following Thomas it seems that even if the past could be changed, it does not imply that an event did not happen, but rather that it happened and later it was cancelled.