I'm just curious about when they realized the wedding would still go ahead. If it was touch and go until the last week or so, and then they had to pull something small together quickly and just hope that their very closest friends and family were willing and able to drop everything and attend, I would expect the HC to be following up with the uninvited original guests soon with an announcement and apology (again, maybe Mom jumped the gun and shared info without their leave).
If it was something about the original wedding plan/planning itself that was causing them strife, and once that was off the table they realized they still wanted to get married but on a much smaller scale, with several weeks to go, then I think they should have contacted the uninvited original guests beforehand to apologize.
Especially with the original and revised wedding dates so close together, it feels very much like "uninviting" to me, and even if an understandable course of action, it merits an apology, I think. If the couple had said they were unexpectedly pregnant and decided to wait until after the baby was born to get married--several months, after which their lives would be very different--I think it would be much more understandable that wedding plans might be revised (scaled down) and people who were invited to the first event, wouldn't necessarily be invited to the second. Even so, if someone was close enough to the HC to be invited to the first event, I would expect some kind of regretful explanation from someone, even if informal and coming from the HC's mom or whoever the closest contact to the guest is.
I guess that's the bit I'm stuck on. I know there can be people at weddings that the HC don't know well, from a cousin's new boyfriend to friends of their parents. But I think once you've invited someone by name, officially included them in that circle of people who are welcome at your very personal milestone life event, you owe them a bit more explanation/apology for a cancellation (and redo without them) than you would if, say, you cancelled your neighborhood block party.